I spoke with lovely person at NMC today to confirm my child's score. She clarified info about how boarding schools work and I thought it was worth posting here for future reference. See posts under the Scholarship section for complete details of how National Merit process works.
Boarding schools with at least 40% out of state students have their own cutoffs rather than the cutoff in their state.
There are 4 regions:
New England/Mid Atlantic
South Atlantic and Central
Mountain Pacific and Southwest
In each of these regions, the cutoff is set at the highest cutoff for the states in that region.
Hope that helps folks in the future...