I have an off-the-rack waistcoat, that I feel needs to be altered slightly... It's how much that I'm unsure of...

I like to wear my sporran somewhat higher (top edge above the pubic bone), and I presume that the waistcoat should not fall low enough to touch it; but, is the guideline anchored elsewhere, say relative to the waistline of the man or the top edge of the kilt? Do I just have to lower the altitude of my purse relative to the length of my vest?