Pete: If I understand your question you are aasking what size of boarder you should have to put a stich line next to it with out the edging done? I try to round my edges before I stitch and if I run a channel to stich in I usually just make sure I have enough room as to not run off the edge of the leather.
Before you work on a piticular project I would practice with scrap leather, I have ruined more than one project by not practicing.