About 3'' from the bottom, 3'' from the end of the apron is one standard way of doing things. I've never been much on kilt pins, but have worn them on occasion. If I can, I like to find a land mark on the tartan (a nice place where some lines intersect, for example) that's in about the right spot and place the pin over that. It makes the pin look like it belongs there... Just a couple of thoughts.

Also, I'm sure you've heard it before, but be careful of the kilt pin. Only pin it through the top apron and make sure you don't catch it on anything! My dad's red Ross kilt has a nice rip, from just such an occasion.