shadowplane676:
(whether points pulled low at TDC, or a trigger wheel input)
Unless you have a trigger wheel, you will be using the timing from the previous cycle to predict the timing of the next cycle. The engine speed isn't actually uniform, and the lower the RPM the more variation you'll get. The hardest case will be when cranking. To make life as easy as possible, I suggest you position the crank position sensor so that you get a signal slightly before the earliest ignition you ever need to use (for one cylinder). This means that you will only need to predict the engine speed for a very short time rather than for a whole crank revolution. Even so, you should be prepared for quite a bit of trial and error to find a timing setup which will let you start the engine. Once it's up and running, the problem gets much easier.