I use a combination of excel and prior year TT version (desktop - not sure online allows this), to run a couple of estimates during the year, just need to allow for changes in tax laws so deductions/limits are slightly different eg 401K/HSA, but gets close enough using prior year software. You can also do final tax table calculation manually in excel if needed. You can model a W2 based on pay statement and estimate final withholding etc; you’ll need to estimate interest and dividends - most brokerages provide tools to project income based on current holdings but won’t project reinvestment from bond maturities so you have to allow for that etc. Once you have a baseline AGI and tax, you can start plugging in hypothetical 1099-R or cap gains into TT to see the effective tax rates for those and where you may trip up e.g. triggering 3.8% NIIT. There may be other tax tools that are available more timely for 2025 limits/brackets.
Navigating the estimate tax safe harbor depends on your situation as to whether you can use 100% of 2024 and then timing and amount of cap gains or Roth conversion doesn’t matter penalty-wise but may be an overpayment. You can also start out with that method and reduce estimated tax later in the year once situation is clearer, as overpayment in earlier quarters will eliminate underpayment later but you can’t eliminate an underpayment earlier by overpaying later. Personally I did a Roth conversion last year so I paid more ES in Q4 and filed with AI method and was able to eliminate the penalty but found it a lot of extra work (need to do state also). This year I expect my situation to be stable so I’m just keeping it simple and doing 100% of 2024 ES and then doing Roth conversion now instead of Q4.
