My earlier measurement of the Open Pro rim confirming the 605mm ERD was "eyeballed".. I've since used the two 200mm spokes with the spokes threaded into nipples flush with the screwdriver slot, and measured according to Musson's method, the ERD is 602mm. The only way I can explain the 3mm discrepancy is if the spokes were screwed in flush with the top of the nipple instead of the screwdriver slot, it would be 205mm.
The nipples themselves are ~10mm long, which is the reason I suggest rounding down instead of up, especially since double-butted spokes can stretch a little under tension. What you don't want to happen is to bottom out the nipple on the spoke threads before you achieve the necessary spoke tension. If the spoke is just a fraction of a mm too long, that can easily happen.
Here are some photos of the 200mm measuring spokes and the distance between the spoke ends (202mm) with the spokes inserted in opposite holes in the brand new 700c Open Pro rims.
