Depends partly on where the buttonhole actually is. In most kilts, it's between the 2nd and 3rd pleats, but, depending on the size of the pleats at the waist, it could be anywhere from 1" to over 2" into the pleats. Even if I put the underapron strap on in exactly the same place on the underapron, the buckle will buckle into a different hole on the two kilts I've just described. Beyond that, it depends on where I actually put the strap on the underapron and how far back I hold it from the edge. And, of course, it also depends on where the kiltmaker chose to put the buckles for the apron edge as well.

And it depends on how the kilt is designed. I put an extra inch on the fringe edge to make sure that the edge actually covers the underapron; some kiltmakers don't do that.

In truth, it doesn't matter, because the kilt belt covers up the buckles and straps.