See Masonite instructions here.

For a thirty-inch wide door, ro is 32"x82". Remember you need to account for the jambs.

This post was edited by worthy on Sat, Oct 12, 13 at 19:38

83" is good...probably need to add a 1x4 and shim from there.

If the ceiling height is 83" from the floor, getting an 80" prehunhg door in place will be tricky, since the outside height of the prehung frame is about 81" high and most trim adds about an inch and a half to that for a trimmed height of 82&1/2".

If I were framing the RO, I would install a double stud on each side and a single top 2 by 4 flat against the roof(to provide a nailing point for the frame and the top trim.

At worst, the top casing may have to be reduced in height.