Are we talking about gliding for range - or minimum sink?
I feel that the question posed by your FIC instructor wasn't quite the one you've posted here.
For max range in a prop driven aircraft, you would indeed fly at just over min drag speed (to avoid going below min drag speed with the consequent power increase needed to get back on the 'right' side of the drag curve). However, for minimum sink, the glide speed is roughly the same as minimum power speed - and that is about 3/4 min drag speed. As whatunion so rightly says - add a few effects for a real aeroplane and the 70/85 relationship seems fair.