I would think it would be best if the knife were in the center. That makes all the calculations a bit eaiser. The knife you plan on using, is it able to cut in all directions? Kinda like a diamond point or etching pencil? When I think of hobby knife I think of an X-acto type with a razor blade in it. So cutting in any direction would mean rotating the spindle to orient to the cut. Just wondering how you do that.