Fascinating project ! Interestingly, 3.5 milliseconds seems like such a long time, when fractions of microseconds can be measured with micro-controllers.
Another interesting project Ken and a great use for the ubiquitous 555. The arc length-to-voltage demonstration reminded me of the the old TV EHT probes that used to work on this principle.
Thank you for posting! Very interesting. I will try to adapt your circuit to a bench test bed for old dwell/tach meters. I have 6 cylinder vintage vehicles with 6, 8 and 12 volt systems and a box full of old test meters. I'd like to use this circuit to see which meters are a suitable fit for each voltage system. What dwell angle is your 3.5 ms equivalent to?