Squaring the fretboard end of a headcap tp create a straight nut slot is a minor detail, but important. Typically builders use a variety of methods. Just cut it with a saw, use a disc or belt sander, or sand the edge by hand to get s statraight edge. Some builders cut an angle on the head cap end to accomodate an angle nut face. I cut my ends parr[allel to the top surface of the neck. For a long time I cut it to length and used an angled shim on the disc sander to produce the necessary angle I need to have a perpendicular end at the nut. That works, but I almost always got a slight deviation which caused me to make a nut that was a bit thicker on one end than the other. I do better with this handplane method.