The rough framing for the shower will be determined by what the Mfgr. of the shower states the framing and RO size (rough opening) of the shower needs to be. When finished you need to measure all walls, floor and front opening from corner to corner , like a square with an X in it, to be sure it is true. the lenght from corner to corner should be the same for both measurements of each area you measure and to Mfgr. specs.