i am doing an addition with a full bathroom that is slab on grade. we are getting ready to start the groundwork but havent selected our fitures yet. i think all tub/showers may have the drains close but not exactly in the same spot? can i safely complete the rough in ground work prior to picking out my new tub or do i have to select my tub first? if i dont have to wait then what is a safe dimension to center the p trap off of the rough framing?