MOR
I too am in a commercial jet but not too long ago flew turboprops into AP every day, from the left seat.
Yes there are set tolerances but having seen the major distortions on the approach... on many occasions. i stand by my comments, The point i make is that if you were in IMC you would have no idea of the error. Especially on a single NDB DME approach you have no secondary data to back up your information. How would you know theres anything wrong!!
Yes fine to say do the GPS approach....If you have one... and why should you have to have one. The NDB approach should be adequate.