Wheelbuilding / Wheel Building / Wheel build help

Yes, you have 3 numbers, known bead seat diameter (BSD) & your two depth soundings A=depth to bead seat B=depth to spoke bed.

ERD + 2B = BSD + 2A = overall diameter of rim

Thanks for that. Do you always round down? If so it looks like I need 20 290 and 24 284 cx ray...