So for me. The back plate is the reference, and it HAS to match your calculated values most importantly for the X width so the X linear rail fits. That back plate is the most important and should be the most perfect thing you can get, make sure the diagonals are square.
Then center the Z rail in that back plate.
From there each side gets added, check the diagonals and make sure they match. For me this sometimes means leaving a small little gap.
Really it is a game of checking diagonals a million times. There are a lot of diagonals. Also widths need to. The back is the reference. If the back is 200 wide the front, top and bot all need to be 200.
Last is set the side Z rails in and make sure they are measured from the back plate, and top and bottom are equal.
Does that help?