One (1) U is not equivalent to 1". It is = 1 and 3/4 inches. Do the math. Also, although 19" racks are standard, you have to add 1/8" on your rack space to fit almost most pro racked equipment. I prefer using wood ears as they are non-conductive, preventing the possibility of all the equipment being part of a ground loop.
Hope these help. Good luck in your rack building.