Get two spokes you know the length of with ideally the nipples you'll be using, spokes through opposite holes on the rim, nipples threaded to right place, measure gap between spoke elbows with Verniers, add gap to 2x spoke length.
Doesn't that depend which nips you're using? DT Swiss and Sapim nipples are different IIRC, although I can't remember which ones have threaded sections which vary with nipple length.
Get two spokes you know the length of with ideally the nipples you'll be using, spokes through opposite holes on the rim, nipples threaded to right place, measure gap between spoke elbows with Verniers, add gap to 2x spoke length.