How much should I pay my employee who is out on COVID leave?

Under the Families First Coronavirus Act, full-time and part-time employees are eligible for different pay when they miss work for COVID related reasons.

It is important to note here that the standard for FFCRA pay for a part-time employee is the number of hours they are "normally" scheduled for during the time period over which they missed work. This is because you may end up with a situation exactly  where you don't put someone on the schedule because you know they won't be available (like because they are in quarantine). In these cases, you would still pay them for the hours they would otherwise have been scheduled to work.
Full-time employees are eligible for 40 hours per week, up to 80 hours of total FFCRA pay.