A alternative to the fixed position would be to use a two part site where a set of marks on one must line up with marks on the other.
Not sure if I am explaining that right or not. It may not be as precise as a fixed distance but it would allow for a few different mounting options (or using it free-handed) without having to deal with calibration issues.
Originally Posted by Lohman446
Page 8 of an MCB thread
We are lucky to still be talking about paintball.