Measure the ERD of the actual rim you own. Take a pair of spokes of known length, stick them in opposite holes, thread the nipples on as far as you'd like (some people aim for the slot, some the nipple top), measure across the elbows with your calipers, and add to twice the spoke length. Do that in a couple places (some rims are a little ovoid, the extrusion thickness can vary, etc) and do something sensible (ex - if you were aiming for the slot you might take the minimum) to reconcile any differences.

Dump that into the spread sheet of your choice.