If you use one, you should put it on at the hip measurement you made the kilt to. But honestly? I would try to talk a guy out of a hip buckle if he had a large waist. In any event, what you do not want to do is put the buckle on where he could buckle it so that the apron pulled in under his belly (hence why you want to put the buckle on at the over-large hip measurement that you used for the kilt).