think of where an engine starts its cycle. that point is 0*. every 90* is a reference point for the injector timing (hence 8 reference points). a 5 in the box would be 450* degrees through the cycle
try this... drop all of the formula except normal(90) and it will give you the exact point in the cycle where the injector stops firing.
5.55(90) = 499.5* through the cycle.
the formula is to base it off cam rotation not crank rotation.