120 degree. The 100 degree is for the AN426 rivets.
I believe it's mentioned in one of the first chapters of the manual but my manual is in the workshop.
If you really want to get the heads flush, consider doing the final drilling using the 120 degree cutter with a backing plate. For instance, final machine countersink the box spar assembled so that the pin of the machine countersink is held centered by clecoed plate behind.
