First of all, MSFS does not model the PT6a correctly, in fact it really doesn't model it. With that understanding, you need to set the condition levers to low idle and keep them there. (flew with 2 different companies having a fleet of these on different a/c and never heard of anyone using high idle, BYW, applies to the twin otter, not alll a/c with PT6's). In actuality you should only have temp problems on hot days- typically summer. Expect to be torque limited. Prop rpm? Depends on the a/c: twin otters we used 75-80% rpm always, unless an engine failed- then full prop. At the other company we flew B99's, and you need props at 100% for take-off, don't remember the figure for climb, and another reduction for cruise. Approaches were done at cruise prop setting, full forward on touchdown.
FSX? Too bad. Try to find FS9.
Bob.