Jose wrote:

What is the protected area under an NDB approach course? Since the
course guidance is cruder than an ILS, I'd expect it would be wider, but
how much wider? I ask also in the context of when I should consider
myself "established" on course (for purposes of descent).

Jose

From TERPs:

b. Area. Figure 57 illustrates the final approach primary and secondary
areas. The primary area is longitudinally centered on the FAC, and is 10
miles long. The primary area is 2.5 miles wide at the facility, and
expands uniformly to 6 miles wide at 10 miles from the facility. A
secondary area is on each side of the primary area. It is zero miles
wide at the facility, and expands uniformly to 1.34 miles on each side
of the primary area at 10 miles from the facility. When the 5 mile PT is
used, only the inner 5 miles of the final approach area need be considered.

Also, the minimum obstacle clearance is greater than other NPAs; 300
feet with FAF, 350 without.

The FAA has never defined on course. That is up to the pilot. The IR
PTS gives some fences, but that is about it.