Spoke length is a three-dimensional problem: the spoke runs from a hole in the hub flange, sideways to the rim centreline and around by however many crossings you lace. Get it wrong by two millimetres and the nipples either bottom out or leave threads showing. This solves the real geometry for both sides of a dished wheel, and then tells you the thing most calculators leave out — how badly the dish has weakened your left-hand spokes.