I can't seem to find the conventions / standards for roughing in water / drain lines for say a 5 foot or 6 foot vanity with two sinks. How do you calculate where on the wall you want the lines to emerge?
first thing we need to know is which is it 5 ft or 6 ft
if it s a five ft which would be 60 inches divide that by 2 which is 30 inches. go 15 inches each way from the center and that centers your waste pipeon each sink bowl. I usually put my vanity waste 19 and a half inches off the floor. as far as the water line goes measure 4 inches each way from each drain pipe coming out the wall and I usually put them 22 inches off the floor. But these days with the flex connectors some people put them a little lower to have easier access.
On the 6 ft divide 72 by 2 to get the center of the cabinet which is 36 in. then measure 18 inches each way and that gives you the center on each bowl and the water lines are the same as above. hot is on the left cold is on the right.