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just measure tip to tail and divide by two, that's dead center. Also, when you get them mounted make sure the shop tech measures it too - I usually ask them to measure and mark it while I'm there so they're mounted properly.
zero is probably core center, they're are a lot of factors that go into core center so it's hard to tell. Also, it could be that the graphic is slightly off so its center on some skis but not on others - that's why I always measure cause the graphic might not be exactly at center even if it says it is.