I'll start by saying I've only done helical because that's what clamp came with the jig I have. What I do on my jig is set the angle so the whole base of the feather makes good contact with the shaft. I found on the website of a well-known fletching company (I'm not sure I can say where or post a link, as I'm not sure if they're a sponsor) that the optimal positioning for a helical fletch is 1/16" offset from the front to the back of the feather, which is about 0.75 degrees. These numbers are using a 5 inch feather. I hope that helps.