They're torpedo tapered, so they're parallel half inch for about 2/3 of the shaft, then they drop down to 3/8 at the nock. They're all slightly different, because I was tapering them to match weight.
I've had a go at cutting the fletchings once they're glued and just can't do it. I find it completely impossible to keep the cut straight for 8"! I do all of mine using the tried and trusted tape method - split the feather, grind the quill etc etc, then stick wide tape to a straight edge, stick the feather to the tape using the edge to keep it straight and pull the tape off the edge. That way you've got a perfectly straight feather ready for marking out and cutting, and the tape holds everything in place. Sharp knife gets it all done in a heartbeat, and each one is identical.