"offsets" =/= "you are no longer in the hole" for anyone wondering if adding a governor means making tax bonus buildings worthwhile. The specific mechanic is that Regional Governor etc multiplies your tax income. Whereas the building's bonus adds linearly or something.
Tax screenshots.zip Screenshots
Pergamon, a settlement "in the hole"
With governor: 2556
and with tax bonus buildings: 2556
and with tax penalty buildings: 2556
Without governor: 1461
and with tax buildings: 1461
So this governor multiplies tax income by 2556/1461 = 1.749 (75% bonus)
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Sparta, a settlement not "in the hole"
the "tax bonus buildings" give 20+10+10+10=50% of building tax income bonus%
With governor: 34134
and with tax bonus buildings: 35698
and with tax penalty buildings: 33897
Without governor: 21604
and with tax bonus buildings: 22594
and with tax penalty buildings: 21454
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So in Sparta (since it is the only one showing differences)
The governor multiplies tax income by 34134/21604=
1.597, or a 60% bonus
The upgrades give:
WITH governor 35698-34134 = 1564
WITHOUT governor. 22594-21604 = 990
So... 1564/990 =
1.597
What about the ones with tax penalty?
WITH governor: 34134-33897 = 237
WITHOUT governor: 21604-21454 = 150
So... 237/150=
1.58
Clearly the governor is just
multiplying the tax income by 1+ (his bonus). It doesn't act in the same way as a building's "tax income bonus" does.
I can move someone to a settlement with nothing but just a governor's house or close to it, the effect is the same. (Oh, it'll automatically be in the hole, as they have the penalty though, so basically the same as Pergamon)
Oh, yeah you can also see that my pergamon guy would be better situated in Sparta, his extra 15% multiplier would be worth 3000 more income. Wow. That's half a stack right there. I suppose it would be best to check the bonus for each general and then reassign the top guy to Sparte ...
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As a side note, the way a positive and negative building tax income bonus interact is unusual and not what you'd expect. Building a +10% and then a -10% results in a lower tax income, so I'm not sure about that.