Actual elongation under load is minimal. In any case spoke length calculations have never factored it, so measuring this way is equally accurate to mathematical calculation. Mechanics have been measuring replacement spokes this way for a century or so. You'll also note that I subtracted 1mm targeting the height below the top of the nipple, which would leave room for elongation.

The only difference between Andrew's method, and mine is the elimination of the need to guess the rims thickness. Either is fine.