Taxes are calculated as a percentage of what in SimCity Buildit?

I know that the tax rate varies based on how happy the Sims are. Let's say my Sims are 99% happy. The tax rate is 20%. But 20% of what?

City hall says I get a tax rate of 20%, which equals §12075 every 24 hours.

If the tax rate were based on population, my population is 104184, so times 20% = §20837. That can't be it, so it must be something else.

Solution 1:

Here is what I received from EA:

Please note - the figure of 20% tax is calculated from levels and ratios for happiness and population, it's not as simple as happiness multiplied by population! A 20% tax rate is not simply 20% of your total population number in Simoleon form. If it were this simple an equation, early level Mayors would receive barely anything and find it a real struggle to make money for a long time, whereas higher level Mayors could buy everything in sight and would dominate the Markets. We didn't think this was very fair, particularly for a free-to-play game, and the balance of the game would have suffered too much.