@JohnB5677 Thanks for the detailed reply. I guess my main question first, before I do anything else, is: Do I really have to do the "what if" (as you put it) tax return to figure the estimated taxes, in order to meet the safe harbor requirement of paying 100% of last year's taxes? Or, in a situation where a spouse dies, is it okay to base the estimated taxes on the past year's joint return, filed by a surviving spouse?
It's not really important for my mother, whether or not she accurately estimates what next years taxes will be (my parents taxes tended to fluctuate a lot in the past, based on a lot of factors). The point is just to avoid a penalty by meeting the safe harbor requirement.
