Your best glide speed is for maximizing range, not "still air". The point of a power off 180 is not to maximize range. You might have to if you mess it up, but that means you are flying on the extreme of being low and barely making it, rather than "bracketing" as described above. Your CFI should have given you techniques to come in neither high nor low, but in a position where you can get higher or lower on GP if necessary. CFIs really got to be held accountable here, this is usually one of the weakest areas, and it's passed on from CFI to CFI as "guessing" and it screws students. It can be done methodologically and quantitatively. After all, what if you have to actually land engine out?