Originally Posted by FBinNY

Decide which holes in each flange will have the spokes going in the most opposite directions. Measure the distance apart of these holes (across centerline of the hub). Subtract from ERD (allowing for nipple height), and divide by two. Now add 1mm for every 10mm of CTF measurement, + another 2mm for good luck (accounts for over/under lace, and other stuff) and you have a spoke length that should work.