Protection ranges for NDBs are only guaranteed by day (and therefore not at night).
I have been trying to locate the reference in the UK AIP but I recall that if you are within the protection range (by day) this guarantees that the strength of the desired signal will exceed the undesired signal (i.e. other NDBs on the same or nearly adjacent frequency) by 2 to 1.
I can't see that aicraft height is a significant factor since it is normally the "ground wave" which is being used, although "sky waves" become a major factor at night (which is why PRs are not guarenteed at these times).