The example I am trying to explain is if your Handling Pilot was suffering from "a lack of ability" or wind* (or the hand of god), and failed to fly directly over the Tacan
He would have to grossly f*ck-up. For example at 8 DME an angle of 5º equals about 0,7 Nm. To end up more than this 1Nm before the station he would have a tracking error of about 45º.
If he can not track to the station more accurate than 45º I can't see how he will manage to know when he is within 5º
P.S. You can not replace the example NDB procedure with a TACAN since A tacan station will normally not be used as an IAF, in case of a TACAN approach the IAF is allways a radial/dme fix (as I mentioned before)
Greetings, Bart