Replying to How to determine mount position

So I just got my new gear mounted at a local shop. Atomic Punx in 182 with Salomen Sth 13's.While I was going through the process no one asked for mount position. I remembered just before I left and quickly told them to center mount without specifying true center. When I got them back I could already tell them weren't mounted right. I talked to one of the guys and he said that it was mounted -.5 from center, but I think he meant from core center. Now I'm not sure how to determine mount position but there is about 26 1/2" in the tip and about 24 1/2" in the tail to the binding. If it really is only -.5 from center then its not a big deal because I don't want to weaken the ski by remounting for such small difference. But since it seems like the difference is larger how do I determine mount position.

Tl;dr: Trying to determine the mount position on my skis. How do I go about doing this?