In the US it probably makes more sense to go with an inch-based standard, since it's easier to find materials in those sizes. I've been making up my own 'standard' for rack-mount electronics with an eye towards making it work nicely with as many other defacto standards as possible (19" racks, standard lumber sizes, pegboard, unistrut, erector sets): http://www.nuke24.net/docs/2018/TOGRack.html