I was taught that a properly worn neck tie should just touch the top of a man’s p@nts, covering the top p@nts buttons at the longest. I carry this etiquette over to kilt wearing as well, with the tie just grazing the top of the kilt or buckle. Because on average kilts are worn higher than pants, this scheme can be a little hard sometimes with standard neckties, they can be way too long sometimes and I'll end up tucking a bit of the narrow end into my shirt. I've aquired a few of the shorter wool ties sold through various kilt vendors, and these are better for me.

This having been said, I find I rarely wear a tie without a waistcoat, so that covers any miscalculations in length I may commit.