Hello,
I was always taught by my Da who was taught by his Da ( frae Scotland, who was taught I assume frae his Da, you get tha point) that 2 inches over frae the fringe and 4 inches up frae kilt selvedge tae the bottom of pin. Also, has others have already pointed out, the pin only goes through the outer apron. Reason being, if you pin through both aprons their is a high risk of damage to your kilt and we certainly do not want that to happen. The pin is essentially there for a wee bit of weight to keep the apron down in case of a Marilyn Monroe moment ( strong breeze lifting the aprons up exposing the secret ), but typically with a good heavy weight tank the issue is rare and more so a fashion element of which I am guilty of as well . Of course you could always just wear it any way you please . I hope my suggestion helped you though. Take care and be well.