How low should it go?? Sporran that is.

[PiobBear]

I was always taught a hand's breadth beneath the belt buckle. However, I've found that being a bit bulkier than a traditional piper, I wear it better a bit higher than that, so it doesn't accentuate that feature of my anatomy.
As for sitting, I spread my legs then close them, to draw slack into the aprons. The sporran only comes into play with my horeshair.

[RockyR]

I was always taught a hand's breadth beneath the belt buckle.

This is how I tell people to wear it... 4 fingers below the buckle as a rule of thumb.

[Kilted KT]

This is how I tell people to wear it... 4 fingers below the buckle as a rule of thumb.

that's the rule I use. Any further down and my legs get a beating. Highter and it winds up being a second belt.

[Andrew Breecher]

I simply wear my sporrans where they feel comfortable. The "hand's width below the belt" is a good starting point, and adjust from there. My different sporrans each have a "sweet spot" where they hang, and they are each a little different, since they're each shaped differently. I have a different sporran strap for each. I've made use of a leather punch to put buckle holes in exactly the spot where they're comfortable.

Andrew.

[Nanook]

I simply wear my sporrans where they feel comfortable. The "hand's width below the belt" is a good starting point,

The problem is: "Where does the belt go?" :-)
Seriously, not all kilts have the same rise and hang. Its typically an aesthetic selection: the sporran should be centered, squaring the area between hips, arms and knees and the chain into a "golden rectangle" with the strap hanging from the hips should form a "smooth" arc. This is, in general, a good 4 fingers or hand's breast below the bottom of the waistcoat or belt but that measure only really makes sense when the waistcoat length or belt is so positioned, viz. the waistcoat length is so selected according to the golden ratio of 1:1.6. This ratio has been considered as a kind of "divine proportion" over the past 4 centuries. Within clothing the center is the naval (belly button) and its considered at its "ideal" following this proportion. (I'm not joking!). Clothing design then balances out from the naval and follows these "golden rectangles" (again, I'm not joking) looking to create balance or imbalance.