Cheers i was more interested in it for modding purposes, specifically i've got a race idea running round my head that will get a bigger than normal chunk of income from tourism, but figuring out what numbers area balanced is a major PITA when you have no idea how values are derived. 

There's precisely 1 reference to tourism in the global settings - <GalacticTourismSpendingMod>. This is the only adjustable setting in the calculation itself, prior to the bonuses from tech etc. It appears to act as a multiplier rather than a modifier - that is, tourism is (rest of the equation)*<GalacticTourismSpendingMod>, rather than (rest of the equation)*(100%+<GalacticTourismSpendingMod>).

It may be worth setting this to 1 just to eliminate a variable from the equation while trying to work out the other factors.

I *think* it's % of the galaxy covered by your influence, but that's based almost entirely on a gut feeling. The base number is likely to just be derived from Base in the map size defs, though again, this is just conjecture. One thing I would advise generally is to reduce tourism itself from 0.25 to somewhere around 0.10, then then increase the impact of tourism buildings by a factor of twenty.