Info given to me from a Bell test pilot many moons ago,
Desired airspeed for minimum decent 52 KIAS
Airspeed for max glide 69 KIAS.
The 206 was easy the bigger machines and the 4 blade machines had some interesting figures for weight/RRPM/airspeed, but as said above go by the FM, the info I got was from a test pilot and interesting to work with during test flying