I've used Sigma computers before, and they were always preset to 622-23 (another way of saying 700c-23). But you should check what yours is set to. To measure rollout, I put a small dab of white-out on the tire, then rolled the bike along the sidewalk and then just measured the distance between the white-out marks on the sidewalk. FWIW most computers will be using the metric system on this measurement.

Since the sensor is only counting revolutions, it shouldn't matter where on the wheel it's mounted. I've seen recommendations to mount it as close to the hub as possible, because that's where the velocity of the magnet will be lowest for a given speed, to it should be able to pick up each revolution better, but it's never made a difference for me.

JB

edit: OP, did you mean that the sensor is too far from the spokes (and thus magnet), so it's not picking up each revolution? That is an easy fix - the sensor should be mounted on o-rings or cable ties - just move it in a little. Or move the sensor and magnet down closer to the hub - that's where the spokes will be closest to the fork.