How do you correctly estimate taxes when you have both W-2 and 1099 income in the same year?

This year I kept my full-time job with withholding and also started a few projects on the side. I've received two payments so far and haven't sent any estimated payments yet. I tried the IRS estimator and a few free self-employment calculators, and the numbers are all over the place. From what I understand, for the self-employment piece you look at net profit, take 92.35% of that as the base, and apply about 15.3% FICA. For underpayment, you can set a target using the safe harbor rules (100% of last year's total tax, or 110% if your income is higher). But when you juggle both W-2 and 1099, it's easy to get confused between existing withholding and what still needs to go in quarterly.

If you've dealt with this combo, how did you land on a reasonable payment now without tying up too much cash and without penalties later? I also spoke with Tax Law Advocates to get clarity on what to track during the year: they suggested calculating SE tax on 92.35% of net profit, keeping a simple income/expense log for deductions, and using a W-4 adjustment as a buffer instead of dumping big sums into 1040-ES all at once. They also walked me through checking whether I'm on a safe trajectory given my W-2 withholding so I don't end up paying twice. I'm not planning to fall behind, but if it happens, they said a payment plan can be set up and in some cases penalties can be reduced, though it depends on the actual numbers.