few thoughts:
- the safe harbor for estimated tax is smaller of 90% of that year's tax, or 100% or prior year's tax (110% if AGI will be over 150k which sounds like may be the case for you). So if this extra income is a new gig since last year or you expect sufficiently increasing year over year you may be able to pay ES based on 110% of prior year tax in equal installments.
- if that doesn't help and you're better off paying based on current year's tax, in terms of how much / what % to pay - you'll need to figure out for your situation given the other 'known' or more predictable stuff in your return (e.g. W2), what tax bracket this variable income is going to hit and its effective tax rate (e.g. 24%) - which may change as this income and your projected AGI grows thru the year and adjust accordingly if you hit a different tax bracket or anything significant phases in or out (e.g. the 3.8% net investment income tax on AGI > 200k if single filer) . Personally I use TT to mock up some draft returns to get a baseline even if it's not quite the right version for the tax year it gets in the ballpark and then tweak for changes in deductions or tax tables. Plug in different freelance income 25k, 50k, 75k etc, and see the net impact on your 1040 (similar for state tax also).
- you will likely need to use Form 2210 during the filing to specify the uneven earnings and estimated tax payments to avoid a penalty for underpayment earlier in the year, as the default assumption is this is being earned and paid in equal quarterly installments. Note it's not quite a quarterly process, the cutoff dates are 3/31, 5/31, 8/31, 12/31. This FAQ page is helpful https://www.irs.gov/faqs/estimated-tax
Not a CPA/expert just my two cents how to approach this. Hope this helps.
