Since the print volume size is driven from the rail length, X size = rail length - 100mm. Example, a 300mm X rail gives you 200mm printable.
Y size = Y rail length -50mm. Example, 250mm rail length gives you a 200mm printable.
Z size = Z rail length - ~50mm. This one is approximate because bed mounting is somewhat variable. Original CAD has this at 250mm giving a 200mm cubed volume.
From memory, extrusion length for Y axis and Z upright extrusions are the rail length + 95mm, and the X extrusion is the X rail + 70mm so the example extrusion cuts are 345mm for Y and Z (the uprights that the Z rails attach to are 305mm) and 370mm for the 3 extrusions in the X axis.
Therefore the extrusion cut list for a 200mm cubed build is:
3X 305mm
8X 345mm
3X 370mm
Mine is a 250mm cubed build, so I add 50mm to every one of those.