I've gutted my bathroom and framed it out. The plumber came and set the tub. I'm getting conflicting ideas about where to set the first sheet of durock. How much space above the tub should i leave between it and the durock? Should the last course of tile be flush with the bottom of the durock? I'm being told i should fill the tub with water before I caulk between the tile and tub. That makes sense so the caulk doesn't pull away. Any help is greatly appreciated.