I've found that I also have to factor in the weight of the kilt pin. The heavier the kilt pin the more towards the center and higher so it doesn't "stretch" the material.

I also like to find a place where the tartan patterns intersect so the pin looks a bit like it belongs there, rather than being thrown at the kilt to land wherever.

Best system for me has been to look at the tartan and the pin and sort of move it around like a dowsing rod...eventually the kilt pin sort of finds the right place...you'll know it by the feel and the look.

Then double check after you don the kilt because the right place when the kilt's laying on the bed might not look right when you're wearing the kilt.

Play around. You'll find the spot.