I made a neck angle gauge of sorts, gluing a 5/8" piece of wood (rounded on the bottom where it contacts the head) to a straight length of maple 26 1/2" long. It's easier to hold the neck and the gauge than it is to try and use a yard stick and a bridge, which is what I'd done for years.
3 degrees isn't much to measure but you can get an adjustable protractor, just about every hardware store has them in the "precision" measuring tool section for about $12. They work OK but are hard to read and use accurately but they will give you an idea.
edit - That's the one BG, thanks.
Next way is to calculate it with an on line triangle calculator for those not wanting to learn the math.
While this one isn't absolutely "perfect" it is so close the error is negligible:
If you plug the desired angle (3) in the spot for "angle A or B" and plug in the length fingerboard (FB) for "side b" (call it 19, for 19 inches) in a Mastertone, and then hit the calculate button. It will spit out a value for side "a" of 1. Since "side b" was input in inches the result for "side a" is inches too. So if you lay a straightedge along the top of the head or tone ring and measure down to the top of the fretboard at the nut that should be about 1" for a 3 degree neck angle. That depends on the top of the fingerboard being dead even with the top of the head or tone ring at the heel end. If not you'll have to add or subtract from "a" whatever the fingerboard is above or below the head.
The calculator can take several sets of data to calculate the rest of the values. If you know the drop of the FB from the plane of the head or "side a" above and the FB length "side b" and plug them in it will give you the neck backset angle. If "side a" were 3/4" (0.75) and "side b" 19 inches, neck angle would be 2.26 degrees or about 2 1/4.
Hope no one is more confused than they were before.
Thanks, Brian. Your's and Banjo-Joe's seem to work off the same principle, measuring a point on the banjo relative to a level surface, Wyatt Fawleys idea of building something that can be used long-term seems like a good idea, as I hope this isn't the ONLY banjo I will ever assemble or build.
I use the same tool beegee uses.....I've cut a block with a 3 degree angle that I keep handy to set the table saw blade to. With the block I guess it could be a little =/- so I cut all my angles on my necks with the same set....I have four, three degree cuts on the necks for my mountain banjos. harrisonsbanjos.com