Use the point-to-point mileage XXX-YYY-ZZZ only when you change flight numbers or days in YYY, otherwise it's XXX-ZZZ.

While the great circle mapper is an incredible tool, it doesn't always return the exact mileage used by the airlines in computing distances. Remember that the surface miles between open-jaw points counts too. Best to put an itinerary together then phone it in to the airline to see if they count the miles the same as you do, and don't try to cut it too closely.