You're assuming that you can just choose how you go about making decisions every time you make a decision. If you're not granted that assumption, Furcas's analysis is spot on. Two-boxers succeed in other places and also on Newcomb; one-boxers fail in many situations that are similar to Newcomb but not as nice. So you need to decide what sort of decisions you'll make in general, and that will (arguably) determine how much money is in the boxes in this particular experiment.