TurboTax will calculate the unrecaptured Section 1250 gain for you. That portion of the gain is taxed at ordinary income tax rates up to a maximum of 25%.
You can make an estimated payment (or payments), but you can avoid a penalty if you do not make estimated payments if:
- Your filed tax return shows you owe less than $1,000 or
- You paid at least 90% of the tax shown on the return for the taxable year or 100% of the tax shown on the return for the prior year (110% of prior year tax if the prior year's AGI was over $150.000), whichever amount is less.
See https://www.irs.gov/payments/underpayment-of-estimated-tax-by-individuals-penalty
