FWIW, I prefer my kilt to fall at the bottom of the kneecap. I usually measure from where I wear my pants (and shorts) to the center of the kneecap, but I end up wearing it a tad lower (on my hips) and the length seems about right for me. I have tried pulling it up higher (near the ribcage) and it feels really odd to me, both at the top and the bottom.

I think it really is a personal preference thing. Do what feels best for you and don't worry about what everyone else may think! If you want to see some examples of various lenghts, look at some of the pics on my site (link below).