baseTaxRatePerPopulation 0.025
incomeSpeedData-Slow
incomeRateGameSpeedScalar 0.0
incomeSpeedData-Normal
incomeRateGameSpeedScalar 0.15
incomeSpeedData-Fast
incomeRateGameSpeedScalar 0.30
incomeSpeedData-Faster
incomeRateGameSpeedScalar 0.75
At normal, your capital starts off at 12.6.
((12.6/1.15) -4)/280=.0248
Allowing for the inaccuracy of the display(it's rounded) this equation bears out with the values.
Income is population times the tax rate, multiplied by 1 plus the scaler. 40 population is 1 credit of income, but it's at slow, not normal.