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What bindings do you have? Unless you have an unadjustable binding just mount center and adjust it as you grow. If you have pivots or something might want to get some new bindings if you’re that worried about growing.
What kind of bindings do you have? Most bindings can adjust 20-30mm which effectively means that they can adjust to 3 different boot sizes. So I'd mount the bindings with the heel piece at the forward most position on the rail, that way you can grow without having to remount.