Recommeded center is an oxymoron because there is only one center.
If you want to center mount them get a measuring tape, measure the all of the ski, divide that length in half and mark center.
Put tape there or sharpie, just make an easy to spot, clear mark where center is. Bring your stuff in to get mounted and tell them to mark right on your spot that you marked. Even write down to mount on that spot.
If you do that it is a guarantee that your skis will be mounted dead center instead of being mis-mounted.