Did you have the insurance company hold tax, and if so how did you figure out how much to hold back?

I'm working on my form to send into HR right now and have no idea how much to do! I'll be out for 16 weeks total with most of it as PTO, but 6 weeks will be STD (at 60% of my regular pay). I'm assuming I should hold out the same amount that would have come out during those six weeks (adjusting for the 60%), but really have no clue.