Quarterly estimated payments and self-employment taxes

My husband transitioned to being an independent contractor rather than a W-4 employee this year and we know we need to pay estimated quarterly taxes. Our understanding is that we can avoid an underpayment penalty if we pay at least 100% of our total tax liability in 2024 (divided evenly across the four quarters). 

What we are unclear on is if "100% of our total tax liability" is only the total we paid in income tax OR it includes the FICA taxes (Social Security and Medicare) in addition to our income tax. If it does include the FICA tax, is it only what we paid, or the combination of what we paid and what our employers contributed?

We know that we will have to pay the rest of the self-employment tax and income tax at the end of the year when we file our return. We want to know the minimum amount we can pay quarterly while still avoiding penalties.  Due to our employment situations, it is very uncertain how much we will make in the next three quarters, which is why we would like to base our quarterly payments on last year's tax.

Thanks in advance for your assistance.