Just don't think about it too much
I'm starting to get a clearer image of the cuts, but I've made two just by whittling away, evaluating the shape as I go. We are talking about something like this, right?Attachment:
HeadstockBack.jpg
One of the tricky things is understanding the two planes involved... the fingerboard plane, and the headstock plane. IMO, the handstop lip should line up with the nut edge of the headstock, when looking straight on at the headstock. Or from the side, like this:
Attachment:
HandstopProfile.png
That puts the lip underneath the nut, thus strengthening that weak area. My first one came out with the lip too far toward the headstock.
Starting from a 1x3" neck blank with scarf joint, cut out the headstock shape, holding the saw perpendicular to the headstock plane. Then draw a curve around the bottom of it where you want the handstop lip to be, and cut the waste from the sides of the neck. Keep a bit of distance away from cutting into the curve, to leave material for a smooth transition down from the lip to the neck shaft. Then start whittling
Maybe start by carving the neck shaft up close to the handstop. Then you can start to dig material away behind the lip curve drawn on earlier and form the transition. The shape becomes more clear at this point. But before you go too far, start to carve the headstock's curve up to the lip as well. It is generally flat across its width, so the drawn-on lip curve starts to get eaten away as you carve. Just keep carving on both sides of the lip, to keep track of where it is.A chisel with bevel side down is useful when carving the lip to shaft transition, but the final refinement is easiest with sandpaper or a round file. I also hollow out the headstock behind the lip using a gouge, to give it a more 3-dimensional shape. But if you use a backstrap, you can't go very deep doing this without digging through it.