[bzbarsky]

> Why not pay your income tax when you know - you get your income

Yeah, I have no idea why gambiting is not doing that.

The rules in the US is that your estimated tax payments have to cover the smaller of 90% of your current-year ongoing (quarterly) tax liability or 90% (sometimes 110% for high incomes, an there are other special rules for farmers/fishermen) of the previous-year tax liability.

So yes, you can make them based on last year's income as as safe harbor if you have no clue what's going on with your income this year for some reason. Or you can just make them based on this year's income as you go.