Calculating long-term capital gains from stock liquidation

If I have to liquidate stock to pay my cap gains but the liquidation of that stock creates additional cap gains how do I calculate how much stock I need to sell? Feels like a spiral.

For example let's say I have 100k of long-term cap gains I have to pay the IRS for 2024 year. How do I calculate how much additional stock to sell? Is there an online calculator or an relatively easy to use formula? I understand that a CPA would normally do this and I do have a CPA do my taxes but I'm trying to learn this for myself so that I can strategize on investments.

Thanks to any who can assist.