"In engineering I would work to 1 or 2 thousands of an inch" Yes that is about right I used to shim steam turbines , and mic the plain bearings in them down to about that tolerance .
yes having plumbing especially the waste adjacent to each other does make it more efficient also use 22 degree bends staggered means less blockages later , I also like to recess wall unit mirror cabinets between the nogs if I can
