I'm with Lewis. When I was first trying to get my toneboards worked out, I set up a mirror and pulled air through the exhaust (sucked air from the end) before drilling a taper. This lets you see how the reed interacts with the toneboard. Most of the time, locking up is caused by the reed laying flat, not raising up. A slight modification in your radii can make a world of difference. Another way to observe this relationship is with a clear barrel and a mirror or video camera, the benefit of this method being that your exhaust taper will come into play.
