When you replace a tub in a tiled shower surround with one of these 4 piece tub/shower surrounds what measurements do you have to take off the old tub so you know that you will line up the new tub drain and overflow to the old tub drain and overflow ?:)

When you replace a tub in a tiled shower surround with one of these 4 piece tub/shower surrounds what measurements do you have to take off of the old tub so you know that you will line up the new tub drain and overflow to the old tub drain and overflow ?:)

Unless you have a special order tub the measures are pretty much the same.
The average tub roughs in at 15 1/2" off the side wall and 1 1/2" off the front wall to the center of the traps raiser. These are off unfinished walls/plates. Good luck, Tom