You shouldn't need to reduce the power, just not advance it any further. There will be climb torque for the PC12, I am un sure of what it is in a PC12/45, but lets call it 3700lbs. As the aircraft climbs the torque will reduce, therefore the pilot will need to push the power levers up to keep the same climb torque ( just has he or she would in a CSU equipped piston aircraft as the manifold pressure reduces). Every time the power levers are pushed up and the torque is increased the ITT will rise, eventually the engine(s) will reach a point where you have increased the power to the ITT limit, from that point onwards the ITT will remain constant but the torque will drop away as torque can no longer be increased by pushing the power levers up as the engine is 'temped out' so to speak. The issue with FSX and its simulation of the PT6 engine is that it incorrectly has ITT decreasing as the aircraft climbs, therefore the engine does not 'temp out' in the climb as torque is increased to maintain the climb setting. This gives an inaccurate indication of climb performance as the engine is able to be kept at its full climb torque all the way into the high flight levels. If it could be altered at that ITT does not decrease in the climb, a much more realistic simulation would be had as climb performance would correctly drop off as the aircraft temps out. I believe the F1 B200 simulates this. The use of engine anti ice / inertial separator also plays a part in this combination of torque/ITT.