I have a 1989 Carrera SS8M with a single BBC and a bravo 1 drive. I need to know the proper way to check the x-dim and what its supposed to be at. Thank you in advance......

I do not no how to check the X dim. I do however know how to check propshaft centerline which will tell you the same thing.
#1 Get the outdrive as close to level as possible.
#2 Use a strait edge and extend it from the bottom of the boat to the outdrive.
#3 Measure the distance from the strait edge to the propshaft centerline.
Most boats in general are 4 to 8 inches. Race boats could be less.

Thank you- Im going to try your formula and see what I come up with.........