ORIGINAL: Straightarrow
Helical is preferable over offset whenever possible. A helical clamp will only fit on an arrow shaft properly, one way. Therefore, there is no adjustment range. Large diameter shafts end up with more helical than small diameter shafts and 5" fletches end up with more than 4".
This is my thinking also. I tried to offset my helical clamp with some 4" vanes and couldn't really get it to contact the shaft the way I thought it should. As mentioned I could have probably held it down but I figured if I want more spin, more control then I would just use a different fletch such as a 5 1/2" magnum feather.