I agree that rules of thumb don't always work for everyone. That being said however, when I put on my sporran in the morning in front of a mirror it always seems to look best when it is positioned according to that rule of thumb.

I would be that had you raised your sporran to be one hand width below the belt at the doublet loops that it would have looked too high because it would have been out of position relative to the rest of the kilt. I think the only way to wear a sporran that would look correct on a piper's doublet would be to be wearing one of those extremely long piper's horsehair sporrans which overcome the higher placement by being so long.

But then, that's just my thoughts.