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Recommended on THalls is -5, that's pretty far back for park skis. It really depends on what you plan to do with them. For purely all-mountain/groomers/carving go with recommended (-5), for purely "100%" park go with true (measured) center, for 50/50 all-mountain/park shoot the gap and go with -2.5 of true center.
I had ar7's which have the same demintions i'm pretty sure and I've had them remounted 3 different times due to bindings pulling out. First 1 cm back then centre and then 2 cm back. Centre was better but honestly I think it was more placebo than anything. Their wasn't much of a difference I found.
Or since he "doesent suck dick at skiing" he wants to mount them a bit back because males tend to balance on their heals, and would like to spin faster with the weight of his ass not dead center. True story.
I have rode many ar7s and thalls. I think they work best at 1 back from true center. Its not a symmetrical ski so I think it kinda makes it harder to ride when i mounted at dead center. Hope that helps man