Good question.
If you paid $30k in 2020 taxes, your quarterly payments for 2021 are $7,500 to avoid a penalty regardless of the income source mix.
So, if you are:
1. Reasonably sure of your yearly taxable income excluding cap gains and qualified dividends, and
2. Have established a non-zero tax rate for cap gains and qualified dividends of say 15 or 20%,
you can add the tax due for any taxable cap gains and qualified dividends that brings you over the $7,500 owed in a given quarter to avoid interest payments.
These are all just approximations to zero in on the true amount due which we won’t know for sure until we run it through TurboTax 2021.
