[jnaas2]

The shower base is measured from stud to stud, they are usually a tight fit between the studs. I normally fur the wall out till its flush with the inside of the shower pan then install a vapor barrier usually plastic that goes all the way down to the rim of the shower base then the backer board is held up a 1/4 inch above the rim of the shower stall to keep the backer board from wicking water up. Most people think the concrete backer board will not hold water but it will, with the plastic behind it any moisture that gets thru the tile or grout will travel to the bottom of the backer board and be forced into the shower base. You can also go to www. schluter.com and check out there tile installation process