Tried the same with the Cessna 172, 3000 ft, ISA, still air, 68 kts glide speed. I did get a 250 RPM windmil speed so this looks reasonable, not the 70% RPM you were suggesting but I assume you were referring to constant speed props and not fixed pitch.
I got 9.25 km (4.99 nm) glide range on the Cessna which is 30340 ft, glide ratio is 30340 / 3000 = 10,1 or 600 ft / nm. Anybody knows the actual glide ratio for Cessna 172S?
Edit: Seems like glide performance should be between 4.3 and 4.7 nm according POH.
