For my last few ukuleles, I used a bolt on neck. The neck had a threaded insert and a 1/4-20 screw passed through a reinforcing block in the box and into the neck. I needed to cut the back angle on the face of the neck that screws to the box, but my old Stanley miter box, doesn't clamp very securely, and I wanted it to be precise, so I made this setup, which is similar to what machinists call a Sine Bar:
I wanted a 1 degree back angle (might go with 2 in later builds, but it worked out well). The tangent for 1 degree is 0.0174, which means for every inch of run, you get 0.0174 of rise. So, for 10" of run, that's 0.174 of rise. I took a scrap of neck stock and glued a spacer 0.174" thick 10" from the end and secured this to the fence of the miter box. This became my new temporary fence for cutting the end of the neck.
make sure the stick you use has a square end
Make sure your miterbox cuts squarely top to bottom. Shim the "floor" if necessary.
Clamp everything securely once you are sure everything is in the right place (don't clamp your neck upside down!!)
Don't cut the end off your stick, especially if you want to use it again. It has to remain 10" from the edge to the spacer!
Now, the difference between 0.174 and 0.1875 (3/16) isn't much. In fact, if you use a bit of 3/16" stock (or a drill bit), over 10" that gives you 1.074 degrees, or 7% difference. Close enough I'd say if you don't want to bother trying to reduce the thickness of your spacer. Obviously, you can use whatever spacer you need to get the desired angle.I guess you could improve on this by using two drill bits, 10" apart, where their diameters differ by the desired amount, say, 1/4 and 7/16. coming up with these setups is part of the fun, after all.
Have fun creating home made music!