Never trust the manufacturer's stated ERD for the rim. Always use the ERD sticks and measure everything yourself. You will never **** up this way assuming you can read a vernier caliper correctly. If not you shouldn't be building wheels anyways.

For example, mavic open sport ERD is about 3mm's off from what it actually measures. Meaning, you won't be able to build a wheel if you use their ERD. Properly anyways.