bmw roof rack configuration

have a question about roof rack configuration. just got a 2nd hand bmw roof box 450 and would want to save some $ by trying to optimize the configuration of the roof rack and box myself. cant seem to find anything on the internet though as to the optimum way to do it. so far, all i know is that it looks nice right in the center of the vehicle, and that it should be forward enough to allow the trunk to open. anything else i should know/do? anybody have pictures?

thanks!

Theoretically you would be trying to center the box on the point of equal fore-aft weight percentage. But that might be physically impossible, and that point shifts with interior load, and this is a non-critical weight increment; so within the bounds of common sense do what looks best.