You can't get any helical with a straight clamp. It gives a straight fletch with offset. He needs a helical clamp to get any twist to the fletch.

I highly recommend the helical clamp, especially if you're going to shoot feathers. The biggest concern is whether the fletching will clear the rest one is using. This is less of a concern with feathers, and not a concern at all with a rest like the wisker biscuit. Most fletch lengths will seat properly by adjusting the offset on the helical clamp.