There is only one way to calculate L4/L8/L9 reroute pay. This can only be done after the trip ends.

Once the (however diabolical) trip ends, it's compared to the original trip, and reroute pay can then be calculated.

The number of times you were actually "rerouted" during the course of the trip is irrelevant to the final pay calculation. It's the "originally scheduled" vs "as flown" that matters pay-wise. Hope this helps.