Like a lot of things in life, the most important value is the one that's hardest to measure accurately: the "effective rim diamter" -- that is, the diameter described by the heads of the spoke nipples when the wheel is assembled and fully tensioned. This is a few mm more than the actual diameter of the rim bed itself because you want the spoke threads to come up into the screwdriver slot of each nipple.
When building up a wheel from scratch I have always relied on the published ERDs of my planned rims. It is difficult to estimate the ERD of a loose unassembled rim to the required accuracy.