CDI programming question?

Im trying to write a code to emulate how CDI works(on a dr125 engine using PIC programming). The purpose of the program is to produce a time delay for spark to occur in the ignition system. As the engine starts, pulses from the alternator embly via the pulser coil(positioned at 36degree BTDC) will be collected. The time from the 1st and 2nd pulse will determined the speed(RPM) of the engine.Based on the RPM, the program will call a suitable delay value for spark to occur. This means that spark will only be produce after the 2nd pulse.In reality, the magnet that excites the pulser can be anywhere when you start the engine.If the1st and 2nd input pulses are for rpm calculation and spark is produce at the 3rd input. Theres 50% chance that it not at compression/power stroke.Since the alternator takes 2 revolution to complete a combustion cycle (induction, compression, power, exhaust stroke). How do you know if the sparks output is at the right moment?(at compression,before power stroke)