Well, I put Ralph's formula into the formula I suggested (adjusted for 28 degrees instead of 40 degrees) and get 0.242 times the hub diameter....which is more than close enough and is better than I can measure

In response to tkg, yes, I understood that most ignitions fire on the falling edge, not rising edge of the Hall switch. I tried to frame my comments, however, in accord with the model proposed by the poster (gpar). Also, earlier this week, I looked at the Hall signal from a 3W and in that case, it was clearly a little more complicated. It was easy to duplicate the signal, but since the magnet was polarized with North closest to the sensor, things were a little different. I was not absolutely sure the rcexl ignition referenced by gpar followed the scheme you use or the one used by 3W; although, I suspected it was the same as yours. John