O.K. I dug out the old Twin Otter manual. Beta range starts at +17° blade angle, the low end of the constant speed range. Above this angle the propeller lever controls blade angle. Below this angle the throttle(power lever) controls blade angle with the latter being the or part of the definition of beta range. Pitch at idle rpm is +11°(beta light illuminates at +9° in other words after you move the power levers into reverse on the ground). Max reverse angle is -15°. Therefore with this engine you can be in the beta range in flight.
By the way, can anybody tell me why on the DHC-6 it was designed with a prop lever/power lever interlock so that both propeller levers had to be selected at or above 91% in order to get reverse(Max 96%). If you forgot to put the props forward on landing you couldn't get reverse. Yet they didn't seem to have this feature on the King Air. What about other PT-6 aircraft.