Personally on the ones I have made I will run my stitching groover down around it then try to get the edge of your border to match how much leather you have left outside your stitch groove. that way its kind of equal. You have to take into account the edging and burnishing.
If I dont use a stitch groove I would take my divider and run my stitch line, probably about 1/8" for a double layer belt then run another one thats 1/8" farther in for the border.
Thats just how I do it, Ive only tooled a couple of double layer belts and went primarily from photos of people with much more experience.I wanted to throw up the links to the belts that I have made since I started.
http://leatherworker.net/forum/index.php?showtopic=17591&st=0&p=113804&fromsearch=1entry113804
http://leatherworker.net/forum/index.php?showtopic=19176&st=0&p=124609&fromsearch=1entry124609
http://leatherworker.net/forum/index.php?showtopic=19970&st=0&p=129306&fromsearch=1entry129306
http://leatherworker.net/forum/index.php?showtopic=20216&st=0&p=130726&fromsearch=1entry130726
http://leatherworker.net/forum/index.php?showtopic=23876&st=0&p=152210&fromsearch=1entry152210