Sorry for posting to your thread late, but I think this an important subject that requires a full-fledged answer.
As you said, your proposed change helps reduce the gap between a tall approach vs a wide approach.
However, it doesn't address my main gripe with Rationalism which is that it is so powerful that once you unlock it, you have no reason to not use the card. When you are going for a science victory, there is no such thing as too much science and there is no drawback to using the card except maybe the opportunity cost of using another card, but since it's one or most powerful ones, it becomes a moot point. Also, as you progress through the civics tree you can use more and more cards and it becomes less of a factor.
Originally in Vanilla, Rationalism provided a simple +100% science from science buildings. The fact that Firaxis later nerfed the card proves that they realized it was too powerful. However, in my opinion, they went the wrong about it, or I should say, not far enough by simply making the bonus conditional and a bit more restrictive.
Before you had no real population target per city. Now, you have no incentive to go beyond 10 population.
You always wanted to get the maximal campus adjacency anyway. Now, you are penalized if you cannot reach +3.
To propose an "enlightened" change to the rationalism policy card, I believe one first needs to understand the factors that influence science output and the fundamental differences between the tall and wide playstyles.
Factors that influence science output
Favors tall
Population-linked percentage modifiers (see proposed changes)
Kilwa Kisiwani (can theoretically boost one tall city more if built in it)
Diplomatic Quarter (duplicates city-state bonus yields in one city)
Favors wide
Science from population (exponential food requirement for population growth makes it easier to get 2 pop 10 cities than 1 pop 20 city)
Flat yield from buildings
Great scientists that increase flat yields from buildings (Hypatia, Newton, Einstein)
City-states
# of campi / Campus production cost formulae (based on tech progression with no malus for previous copies)
Neutral
Geneva/Taruga suzerain bonus
Any percentage based modifier that is applied empire-wide
It depends
Adjacency bonus / Natural Philosophy (it depends on the individual adjacency bonuses of the campi; if all cities have equal adjacencies, it favors wide; however there are more things to consider, see below)
Rationalism (if both criteria are met for all cities, it favors wide; as for the likelihood of meeting both criteria, see below)
Specialist slots (as there are more slots in a wide empire it favors wide, but you need a higher food output to support an increasing number of specialists which is more likely in a tall city)
Playstyles
Tall
Needs more infrastructure investment for housing (granary, sewer, neighborhood, builder charges)
Higher production per turn from more tiles worked allows you to build more stuff per city
You can hard build stuff faster once they are unlocked (such as science buildings).
Each population increase requires more and more food
To reach high population quicker you may need to make internal trade routes
You are guaranteed a +3 adj Campus even if you can't get any adjacency bonus from terrain, but it is delayed from the completion of districts (you can build 6 districts around your campus once you reach 13 pop: city center, aqueduct + 4 other districts; you can however only plan one district that way per city to get +3; to get another +3 district that way would require even more districts and more delay).
Wide
Late game food harvest, makes it easier to reach the 10 population threshold
Higher production output in your whole empire
You have access to more choppable/harvestable resources that you can use to speed up their construction
More trade routes available
You are more likely get more adjacency bonuses from terrain over more cities and a more spread out empire. The adjacency bonuses from terrain kick in much earlier than the completion of adjacent districts.
You are not guaranteed a +3 adj Campus if you can't get any adjacency bonus from terrain (unless you build a district that has no population requirement like a Dam, Canal, Neighborhood, Spaceport to surround your campus by districts)
Note: A wide empire is normally considered to consist of 15+ cities of 10 population, but you could push their population to 13 and get a guaranteed +3 adjacency bonus and it would still qualify as a wide empire (even if there is no formal exact definition)
My proposed change
Rationalism: +10% science from Campus district buildings per population in the city. Double maintenance from Campus district buildings.
Alternatively, this is effectively the same thing as providing a 50% science bonus per 5 population if you want to keep the same spirit of "population threshold requirement" as the R&F Rationalism policy. (ie a staircase function instead of a linear one)
The double maintenance has two functions: it acts a deterrent to constantly run the card and as a higher toll for a wide strategy (or more specifically spamming campi and its buildings in a large number of cities).
Scenario Analysis
With Natural Philosophy and Rationalism (current and proposed change) policy cards active
No Hypatia, Newton, Einstein, Kilwa Kisiwani
2 scientific city-states with at least 6 envoys
Currently
Scenario 1
1 Pop 20 city (10 bpt)
Campus +3 adj (6 bpt; -1 gpt)
Library (2*2+2*2 bpt; -1 gpt)
University (4*2+2*2 bpt; -2 gpt)
Research Lab (powered) (8*2 bpt; -3 gpt)
Specialists (3*3 bpt)
Science total: 61 bpt
Total maintenance: -7 gpt
Ratio: 8.71
Scenario 2
2 Pop 10 cities (10 bpt)
Campus +3 adj each (12 bpt; -2*1 gpt)
Library (2*(2*2+2*2); -2*1 gpt)
University (2*(4*2+2*2); -2*2 gpt)
Research Lab (powered) (2*8*2; -2*3 gpt)
Specialists (2*3*3 bpt)
Science total: 112 bpt
Total maintenance: -14 gpt
Ratio: 8.00
84% more science for double maintenance cost
2 Pop 10 cities (112 bpt) is only 8% less science than 2 Pop 20 cities (122 bpt) for the same maintenance cost
Under Proposed Changes
Scenario 1
1 Pop 20 city (10 bpt)
Campus +3 adj (6 bpt; -1 gpt)
Library (2*3+2*2 bpt; -2*1 gpt)
University (4*3+2*2 bpt; -2*2 gpt)
Research Lab (powered) (8*3 bpt; -2*3 gpt)
Specialists (3*3 bpt)
Science total: 75 bpt
Total maintenance: -13 gpt
Ratio: 5.77
Scenario 2
2 Pop 10 cities (10 bpt)
Campus +3 adj each (12 bpt; -2*1 gpt)
Library (2*(2*2+2*2); -2*2*1 gpt)
University (2*(4*2+2*2); -2*2*2 gpt)
Research Lab (powered) (2*8*2; -2*2*3 gpt)
Specialists (2*3*3 bpt)
Science total: 112 bpt
Total maintenance: -26 gpt
Ratio: 4.31
49% more science for double maintenance cost
2 Pop 10 cities (112 bpt) is 25% less science than 2 Pop 20 cities (150 bpt) for the same maintenance cost
Comparison
Scenario 2 requires more strategic resources to provide electricity, but you are also more likely to have access to more strategic resources over 2 cities than 1.
Scenario 2 costs more, but you can build one more trade route to finance the -13 gpt gap.
Running two of the following cards (Rationalism, Simultaneum, Grand Opera) could actually cripple your economy unless you also run Free Market and build more Banks/Stock exchanges in your Commercial Hubs.
Further analysis
Under the current rules and same variables, 2 Pop 10 cities which only have +2 Campi adjacency (and thus +50% rationalism) produce 54% more science (94 bpt) than 1 Pop 20 city with +3 campus (61 bpt) which is about the same relative differential under my proposed changes. Only now, you should not be able to run the cards all the time.
Even under the proposed changes (which I consider conservative) and the same variables as the scenario above, it would take a city of Pop 40 (113 bpt) to beat the science output of 2 Pop 10 cities (112 bpt).
Under the proposed changes, the higher the population the better the science/maintenance ratio.
Under the proposed changes, running Rationalism makes any city under Pop 10 with campus buildings become an "inefficient allocation of gold spending".
If one were to run multiple of the following cards: Rationalism Simultaneum, Grand Opera and/or Free Market, a 33% boost to the 4 main yields might justify partly the investment of going from Pop 10 to Pop 20.
Currently, it has never been harder in a civilization game to go tall (due to housing caps) and never less rewarding to do so.