As has been pointed out, your instructor is mistaken. The numbers he quoted are not the kind of time series that you would find by testing (like CPorter's times, which I'm familiar with from looking at his curves.)

However, look at it from the point of view of "how bad" the advice is... It's not that bad. For the shot you need N+2, if you develop 300%, you have a fighting chance of pulling out a workable negative. The converse is true, N-2, developed 50% the time, would hold the highlights in check.

And, it's very likely it will be on the test. So go ahead and commit it to memory.