Hello,
I recently bought an old MAC Tools TL-825 timing light. I was puzzled about how the circuit might generate an RPM-independent timing delay, specified by the user in degrees, and so extracted the circuit diagram. The attached document shows what I found, along with my analysis of its operation. Perhaps some will find it useful or interesting.

 

Certainly that scheme is brilliant!! My own attempt at measuring the spark timing angle/valve angle, in an engine dynamometer stand, was less successful and much more digital. It consisted of the trigger conditioned spark pulse, or a mag pickup pulse, feeding a PLL to produce pulses at 360 times the trigger rate. Then the second pulse, whose angle was to be monitored, would cause the count to be latched and displayed. The flaw was that an engine speed was not stable enough to allow my PLL to lock. Possibly a much better loop filter would have helped. What did work was using pulses from the encoder driven by the engine.