I am installing a one piece tub & Shower unit (30" x 60") in a new bathroom I am building in my basement. One end of the unit will be against the end wall and I will build a short wall at the other end. Do I have to make sure there is a stud exactly 30" from the back wall, and if so, is that 30" to the center of the stud, or to the outside edge? I assume after the unit is in place I secure it to the stud walls by using the sheetrock screws thru the sheetrock and the lip on the unit. Is that correct? Is there a web site that explains that in detail? I am doing the carpentry myself and having a plumber rough in the plumbing and install the tub, commode, vanity, etc. so I want to make sure my walls are correct.