Handicapping Riders in a Stage Race

I am organizing a group of 12 guys with varying abilities to do La Ruta in '21. I was wondering how to best use FTP or more specifically , watts/kg to come up with predicted times for each stage. I would then base the competition on time above or below an expected finishing time. I can easily calculate watts/kg, but how would I go about translating that to a predictable finishing time for each stage?

I’m open to other suggestions if anyone has them.