You've answered part of your question already. Measure at three or four points around the rims and average for each method. If you have a LBS you might ask them if they have the ability to measure and if so then ask how much they will charge (and if they do so for little/no cost then buy the spokes from them). I've reversed engineered spoke lengths many times by taking a pre built wheel with the same rim and measured it's spokes and hub info then ran those through the formulas to get the ERD or to get a different hub dimension's spoke lengths.

Experienced wheel builders have gone through this already and have decided on one ERD measuring method and spoke length formula that they have learned produces a certain length that they then add their special "correction" factor to come up with the best length. Additionally different batches of rims of the same model can have the ERD vary slightly. Some spokes stretch a bit more then others and so the after build length can vary some. Published ERDs can be different actual points on a rim or within a nipple too. The Op is at the beginning of this learning curve. Andy.