I would keep away from Science personally. The UA's Spy steals Techs ( Science ), the UB's current draft has a ton of

Science per Spy action and another Spy to gather

Science/techs with, if the UB has some Science as well it'll be too much. 5%

into Science is like 5-10

per city somewhere around there, so it'd still not be very gamebreaking, but I feel like going for more Science is a bad idea. Any other yield would be better.
The problem with the Steam Mill is it's hard to make a very good idea with it. I don't think any yield but

,

or

fits. Maybe

, maybe some

Great People points, but those are less convincing.
Maybe
5

, 1/2 scaler (same)
4

for Manufactory/Coal (almost same, coal +1)
+2(or 3?)
for every tile which contains a Luxury/Strategic resource you have a monopoly of.
No coal cost
This way it'd be pretty strong. Different enough from Waag, because it applies to the tiles. I don't know if it's possible or still too same-ish, but I don't think much interesting can be done with this UB.
Alternatively, maybe something like
X(X?)% of
purchase cost is converted to
in the city, X(X?)% of building/unit
construction cost is converted to
in the city which is pretty much what you suggested, but no Science. Science should probably be avoided.
Just throwing ideas around.