Custom door jamb leg length

Quick question - I’m building a custom jamb for an antique door to be installed in new construction. Already have the head jamb width figured out. For the jamb legs, do I just give it a 1/4-1/2” gap between the top of head jamb and the bottom of rough opening header? At the bottom of the jamb legs, do I bring them all the way down to finished floor height then just use an oscillating tool to cut a bit off to install floors?