It depends your overall situation and how much cap gain relative to your normal tax liability. To avoid penalty you need to have paid thru withholding or timely estimated taxes during the year, the smaller of 100% of your 2024 tax (110% if AGI > 150k or 75k if MFS), or 90% of your 2025 tax - this is your "safe harbor" amount. (See lines 1-9 on Form 2210 where this is calculated when determining whether you owe penalty or not).
Once you determine your safe harbor amount you can determine how to proceed. Often paying ES based on prior year tax can result in overpayment especially if you have high AGI and need to pay 110% of that tax, unless you have a larger income event in the current year which may be your situation. If you meet the requirement based on 2024 tax either thru withholding or estimated tax, then it doesn't matter what income you make in 2025 or when, and will owe the balance when you file next April. If you didn't pay ES for Q1-2 based on 2024 you can still catch those up, there will be some penalty for a few months which may be offset by the interest you can earn by deferring payment on the rest of the tax.
If you need to pay based on 90% of current year tax estimate then you can pay a one-off estimated tax but will need to file Form 2210 Annualized Income method when you file next year in order to show the timing of the income and estimated tax. Otherwise by default, IRS assumes that income was spread out over all 4 quarters, but your estimated tax is hitting specifically in Q3, which would show an underpayment in Q1-2; the annualized method should help reduce or eliminate that penalty. It can be additional filing work as you have to calculate AGI, withholding, divs/LTCG etc by quarter (3/31, 5/31, 8/31) yourself to input into TT.
Alternatively you can still pay quarterly ES baed on 90% of current year tax, with some penalty for Q1 and Q2 but if you catch those up that penalty will stop accruing, just be sure to pay 75% by Q3 deadline of what was due during the year either thru withholding or estimated tax, and then pay the remaining 25% in Q4. Any tax you defer paying until Q4 or April you can earn interest to offset the penalty, so it just depends on your situation what amounts are involved and what makes most sense.
Additionally you can increase withholding if you have W2 income to meet some of the additional tax due tho only 6 months left to do that. Withholding is always treated as timely so it doesn't matter if you pay extra withholding in the last 2 quarters - as long as you don't use the AI method when filing which will show the timing discrepancy not in your favor this time. If you can't meet all the additional tax it would at least reduce the total and quarterly ES required whether you are using prior year or current year tax.
So there's a few options and if you do pay one-off ES be aware of the additional Form 2210 filing required. First step is to figure out your safe harbor amount and therefore minimum tax due, subtract your estimated withholding for the year, and what remains needs to be paid via estimated tax one way or another.
For more info see https://www.irs.gov/faqs/estimated-tax and https://www.irs.gov/pub/irs-pdf/i2210.pdf
Not a CPA. hope this helps.
