Not sure I understand. Isn't MPM in either case the same? It's always going to calculate AMM-SFO or SFO-AMM, which should be identical, no?
Unless you meant that you would have a chance getting overwater. Then obviously it would be comparing MPM of AMM-JFK to SFO-AUH which would obviously differ.
I mean if you add on one leg at a time, then the MPM would change with each leg that you add, as I think MPM is calculated from your origin to destination, and each time you add a leg, your origin or destination changes.
So basically, assuming that's the case, if you backtrack at the beginning of an itinerary (AMM-CAI-AUH) it will almost certainly be over the MPM, as the MPM is being calculated from AMM to AUH and CAI is well out of the way. If you backtrack on the end of a long itinerary on the other hand, you have a good shot of it not being over the MPM, as you got the MPM of SFO to CAI or whatever to work with x1.25.