The straight edge will work on bows with a bit of waggle. I typically heat that out as I prefer a flat level belly. But even with odd areas, you just account for them. On areas that are bending too much, I put an X through that area on the belly. I take wood off the areas outside the X, leaving it alone. Somewhat of an eyeball thing, but just monitor the gap. I have used the outline many times and it works fine. It's just quicker for me to use the straight edge. I do like Jawge mentioned in that I put the tips against the wall and measure to the back of the handle. With any reflex your tips are forward the handle. If I see that measurement start to drop I know I need to assess where. That's when I use the straight edge and compare it against what I had earlier.
