I don't know where the lower threshold number comes from, but the upper threshold seems to directly correspond to the number you chose as the base load required (default is 20) to start opacity. So in messing around I realized I needed to run as Admin AND restart in order to see any effect on a windows 7 64 bit system. My best settings for now (trying to get a more sensitive lower end without losing too much sensitivity on the top end) are the following: 20 0 127 80 254 254 this way at 21 units of force, I begin 1 unit of opacity, and by 127 units of force I am only at 80 units of opacity. Easier to get light strokes that way. Earlier I had set the intial force required to 12 instead of 20 but I found it registered unintended marks too easily which showed up as little "hooks" on the end of my strokes if I drew quickly.