One issue is that random event differences have a significant impact.
Plagues, for example, are one of the most severe negative events - and can happen to you even if you take death-but-not-misfortune.
So the best way to get a real handle on the income is to start a bunch of games with slightly different settings run for X turns, and see how much money you have on average
. Of course this is rather more work.
I had various data on this lying around, it wasn't complete, but I lost a bunch of it in the HD crash.
Also, growth has a big (but in need of quantification) effect on the severity of aging. You might throw fifty elderly mages into the mix and see how they do.
All that said, your math looks right to me.