While I am not a tailor, it occurs to me that as your waistcoats will be bespoke, you can consider your options carefully. Your avatar suggests you may wear sporran hangars as opposed to a sporran belt. How will you wear your sporran when wearing a waistcoat? Typically, one does not wear a kilt belt with a waistcoat, so this may be a consideration. Sporran height (and suspension) may be a consideration when figuring waistcoat "hemline"? Perhaps your daughter might be able to connect with bespoke tailors/purveyors of highland wear for some advice? Best of luck.