Unfortunately, this is something that should be taught to private pilots from day one. What if you have an engine failure? You HAVE to be able to do a power off 180. It should therefore be a basic skill and not wait until commercial to suddenly be important. Remember the point is not to fly best glide speed and go as far as you can, the point is to land accurately. This requires you to do certain things, think about it in a technical way think about how far and low you should go, come up with a procedure that is reliable and adaptable to conditions. If your CFI reverts to "guessing" and can't explain it well, find another CFI and let his boss know. There will always be some judgement and skill involved, but outright guessing and WAG stuff is what you should use to fine-tune it, not approach it from the basic level.