It's not that a good instructor knows everything. It is that a good instructor is prepared for the lesson(s) to be taught that day.

BTW, how many times in flight have you used the radius of turn formula? Even better, how about the ridiculous formula for finding your distance from the VOR by turning 90 degrees from your course...I am not saying there isn't an application for these "tid-bits", but don't saturate yourself with minutia.

The goal is to teach practical and safe flying, that is what people will remember when their brain leaks out. I agree with Photon, in that the more you teach a particular task the easier it becomes to recall everything associated with it. You will be amazed at how much you didn't know, once you start teaching.