Speaking of what other folks do, I've noticed from photo's on the forum that North Americans tend to wear their kilts higher than your average Brit. I'm a bit of a mid knee man meeself and socks just below the knee. To wear my Kilt higher makes it, in my opinion too short. (No critisism here, just saying what works for me).
I noticed too while up in Dumfries recently that younger scots (there were several out and about for the Scotland Brazil game) tend to wear their kilts even lower to the bottom of the knee (ie. kneecap completely covered).
John...I don't know where you base your observations from, but I've observed quite the opposite here in the US. Many guys wear their kilts too long (as has been discussed on multiple threads here about band kilts, etc). I chalk it up to either insecurity about showing a little bit of leg/knee or, more often than not, buying an off the rack kilt that doesn't fit the hips properly and sags down or is too long to start with. Many band members that I've seen have horrible low saggy kilts due to this phenomenon as well.

I won't criticize your preference for mid-knee...perfectly acceptable in my opinion. Below the kneecap is not, though...at least not from my foxhole. I, myself, aim to have my kilts fall at the top of the kneecap. As I understand it, that is the general goal...and if you look at many OLD prints/drawings, it appears that some of the earliest kilts were 2-4" higher than that. I think that's a bit excessive, but I honestly hope we aren't seeing a trend towards "kilt saggerz" that are showing their boxer shorts at the top and have the kilt selvedge at mid-calf level....