This is the 6th set neck I've built and I'm still trying to refine the process. I started out using a router sled and it was just too much setup and produced results that weren't repeatable from neck to neck.
The last 2 necks I made, I rough cut the angle on the band saw and fine tuned it with a plane and sanding block. This worked fine, but setting the angle was still a bit of trial and error.
On this one, I came up with the following. It took all of 5 minutes once I thought out the plan.'
After trimming and fitting the tenon to the pocket, I traced the area where the neck bordered the body.
This gave me the amount of material I needed to remove (A). I transferred this measurement to the bottom (. The point marked zero cut is just that...I don't want to remove any material either side of this point...it's my height reference for the neck so the bottom of the fretboard sits just above the guitar top.
To get the angle I set the neck, fretboard down, onto my band saw table. It's aluminum plate and the flattest surface in my shop. I used a digital angle gauge and zeroed it out on the back of the neck. It doesn't matter if it's truly zero to the horizon or not, it's just a reference. Then, with the heel clamped loosely as a hinge, I slid a pencil under the neck until the gauge read 2.5 degrees.
I used a known good straight edge to mark the 2.5 degree line on the heel.
Then, using digital calipers, I transferred the line to my zero cut point and a point further up the heel and drew my cut line.