I appear to have to throttle back to less than 20% in order to reduce speed in most of the MS Flight aircraft. Is this the norm?

In level flight, the plane should slow fairly quickly by pulling the throttle back. If you are descending, it becomes much harder to slow down.

Good way to descend is to pull power back, holding level flight, trim for the speed you want once you achieve it, and then let the nose drop.

And if you do need to slow down while descending, one way is to enter a slip... this is done by moving the rudder in one direction, while moving the ailerons for a turn in the opposite direction to prevent the plane from actually turning.

 

So, you could put the rudder controls to the left and the ailerons controls to the right, for example, to descend very quickly while still slowing down.