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 Axel
I once saw a guy wearing a Hawaiian shirt, and when he couldn't answer simple questions like the difference between a K2 and a KP2, I followed him around yelling, "Hey! This guy is a big phony!"
That evening, the cruise director gave me a very stern lecture.