In this lecture we are looking at an example of a superconducting magnet which needs a resister to discharge from it's maximum field in 1 minute. Everything made sense until the last line of the image I have attached. We have been told that by rule of thumb you should wait 3tau (3 times the time constant) for the discharge to reach an adequate value. So when the professor says 4.61tau is shorter than the rule of thumb (3tau) and "the rule is on the safe side" I am confused, can someone help me understand this please?