About how much of a turn is needed to make a reasonable reduction in voltage?  A quarter turn counter-clockwise?  More?  Less?  I do not have a meter to measure the voltage directly, but at this point, both because of stability and risk of fire, I'm inclined to start making blind voltage reductions. I just need some indication of how sensitive the trimpot is so I know if I should make tiny turns/adjustments or larger turns/adjustments.

I do have a kill-a-watt, so can measure the power draw at the wall before and after making a change to see how much of a change I made...