From our own Matt Newsome's How to Wear the Kilt -- literally:

"Now you are ready to put on your sporran. Adjust your sporran strap so that the top of the sporran is a few inches (not too much! I'd say no more than 3" or 4") below the belt. If your sporran moves around too much, feels like it is slipping, or feels uncomfortable in any way, you are probably wearing your sporran strap too loosely."

I read somewhere once that a sporran should hang about 3 finger widths below your buckle.
It seems to work for me.