center is center, its usually based on the running length of the ski (there's other types, but this is commonly the one used), so it's always gonna be the running length, divided by 2
now the halos have more rise in the front than the back, and it starts earlier, so when you mount center there is more ski contacting the snow behind you than infront, that's the only downside of the halo it makes for a strange riding position
that being said, mine are centered and i love them