Yes, a bank adds 50% more but of the original. So it is 33% more from the current. And a stock exchange is 20% more than after the bank. This can be useful since it doesn't tell you the base gold you would get without any improvements. Luxes don't help if you can't pay for the bank though.
You are right, that is a useful method. It's drawback is that it only works under a very narrow set of assumptions... namely, that if you have a bank, that you have a marketplace. And if you have a SE, you have a Bank and Mkt. If you capture a city with a bank, you cannot have a Mkt. Or if you sell one off.
Here is a general way (the "
Formula Method" to figure base gold (taxes+taxmen):
divide gold by (1 + 0.5*X), where X is the number of economic improvements (Mkt, bank, SE).
Example: Gold is 90. You have a Mkt & SE, but no bank. Base gold = 90/(1+0.5*2) = 90/(1+1) = 90/2 = 45
If you want to know how much an Economic improvement contributes, simply:
Improvement = Base/2
Or, in English, just chop the base gold in half.
We can also use a general method called "
Table Lookup":
0 Improvement = Total Gold * 100%
1 Improvement = Total Gold * 33%
2 Improvements = Total Gold * 25%
3 Improvements = Total Gold * 20%
The above table is the general case of your method, BTW, except it's based not on the specific improvement, but just knowing how many of them you have (one, two, or three).
Luxes don't help if you can't pay for the bank though.
Weeeellllll, true and not true. If you can't afford to pay for the bank upkeep each turn, it is OK to deficeit spend IF.... IF you are growing, as in a Republic/Democracy. In fact, this very bank may allow the Luxuries rate to be reduced to keep WLT__ coming in a period of growth... or to keep basic happiness when Lux is 10% or 20%. One additional citizen at work pays for a bank's cost. But in general, if your city has 2 or 3 trade arrows... it is not a likely candidate for a bank.
I can still remember being confused about this stuff two years ago when I started learning Civ II, which is why I write a lot more detail for eh newer players.
If you would like to see a striking demonstration of similar confusion, try watching someone (Anyone!
) try and explain the way science is computed in a SSC which has scientists, some/all the science improvements, Copernicus, and Isaac Newton. Probably no one can figure it exactly, but almost everyone will have part of it right.
(the same holds true for thinking of them as little bags of light bulbs and little bags of wonder building concrete) - in short, the most valuable unit in the game.... ummm that is until you discover Economics and they get neat big trucks to drive. 
).
