MSFS does not simulate the"Ground Fine" position of the throttle levers, a position that is over the reverse hump but not quite reverse and is considered a Beta + Power position. You can simulate Ground Fine sort of by using reverse (hit the F2 key a couple of times) to control forward speed. The other option, that we will use in the winter when we do not need the condition levers at high idle for air conditioning is to put the condition levers into low idle like Bob has already pointed out. Just remember to put them back into high idle for take-off and landing or you can get slower reaction time if you need to go into reverse for either an aborted take-off or for landing.
It is normal for a King Air to run away at high idle and the real world airplane will do the same thing, thus the reason for the Ground Fine position.
-KenG
A358 (Retired)