Unless it's a real old model, there are two clamp adjustment knobs. Loosen the set screws, and with an arrow in the jig, place the clamp on the magnets and position it as you wish by turning the knobs one way or the other, making sure the clamp touches the shaft its entire length, or at least for the length of the fletch you plan to use. Then retighten the set screws.
Of course, you need a helical (curved) clamp to impart a lot of helical to the fletch. If you have a straight clamp, you can impart a little offset, but not as much as with a helical clamp because the straight clamp won't rest on the shaft it's entire length, but pull away from it.