Dusty,
To trap the back or belly is your next choice. What material/s your bow is made from should be part of that decision making. If you have a laminated bow with too-strong backing wood as compared to the belly wood, then the back should be trapped (Chamfered). On a wood that is tough on tension as well as compression (Like Osage)then I might entertain chamfering the belly. Per T.B.B.4 pg.118 Trapezoidal Sections, Tim Baker writes that wood is normally much stronger in tension (Back) than compression (Belly). As far as measurements I would mark in from edge with a pencil loaded compass 3 lines, one at 1/8, one at 3/16", and one at 1/4". I would then start scraping towards the 1/8" mark while maintaining about a 45 degree angle on all 4 edges.(You can fade in or out with your chamfer where you see fit), then check tiller and weight after flexing 50 times or so. And if needed I would creep on to the next line with the same approach.
Hope this helps.
Dcoy