Terminal NDB reception range at high FL's
I know NDB reception range is a function of many, many variables, but generally speaking I'd be interested to know how far back of a terminal NDB (ie., one used on an ILS approach course), can you acquire a usable signal when you're high-up in the cruising flight levels? 100NM's, 50 NM's?
Is there a general formula for this?
Maybe a turbine/jet driver would the answer to this one?
Thank you from a instructor.