Take the spoke with the actual threads that will be used - test with the actual nipple that will be used. Determine the minimal insertion allowed without showing threads - and determine the maximum insertion allowed before spoke dead ends.
Most 12mm nipples with box threaded spokes will be 1mm below screwdriver flat and exactly to top of head. That's about 2.5mm of targetted space that is aimed for. With .5mm reserved for overhead at the top in case of error - that leaves you with 2.0mm - 1mm below to 1mm above flat. Hence the +/- 1.0mm the DOS noted already....
Assuming 12mm nipple...use an ERD that aims for the flat - round down a 1/2 millimeter if necessary for the drive side of rear or disc side of fronts. You can get away with a .5mm round up if necessary for non-drive rear or non-disc front.
Having a spoke machine as I do makes things easier - spoke machines tend to add one more thread providing a little more overhead for the same nipple - thus 1mm more room for error at the top side.
=8-)
http://www.mrrabbit.net/wheelsbyflemingapplications.php
Have tools there you can use...
1. However you need to find out where that ERD is aiming and with what size nipple - check the source for that ERD you have.
2. You need to measure your hubs as noted already.
=8-)