My point was simply that you can only know how fast you will lose airspeed if you try it.
Dear Prof., the point is that we do know as we train for it during the PPL and, if we have any sense, we practice post PPL.
We also practice glide approaches, in fact when I operated from Fowlmere, I used the glide approach as SOP for various reasons, so we are familiar with the London Common characteristics of our spamcans with minimal thust - clearly no thrust and a windmilling prop reduces performance further.