The critical thing to me is that the sporran is high enough so it does not bounce on your legs when you walk. It will move some of course but just so it's not down around mid-thigh. Then as TB has said, a few fingers below the belt so it doesn't scratch up your buckle or get scratched up by the buckle.

Variations on where to wear the sporran, wear it on your hip, even swing it around the back if you want. Mount it to your belt and wear it beside the buckle (ie. as Liam Neeson in Rob Roy) The key is Form Follows Function. The sporran is a utilitarian device so whatever works, though in certain venues traditions are good to follow.