Anyway, The step back method is the only way I have found to find centershot. Using one pin, start shooting at 5 yards, stepping back 5 yards at a time. So you should shoot 5, 10, 15...35. If you arrows make a diagonal down the target your spine is off on your bow. If you make a C shape, you arrow rest is out too far. (They will make a gentle C shape). If you make a backwards C then you rest is too far in. Its that simple.
ewolf, I agree with your method, but I wonder about the "C" formed by the arrow. I've never had a bow that was off on centershot that would hit in the same vertical plane at 15 yards, as it would at, say 40 yards. In fact, if I move back to 80 or 90 yards when testing, the deviation from the center line is even more apparent. Maybe, this has to do more with the fact that I never shoot at 5 yards when testing this. I usually pick 15 or 20 yards to start, and then back up to 50 or 60 yards.