Good that there is some non-sensitive area above the "left" mouse button. I find that with the Evoluent, I tend to revert to hold my index finger on top of the mouse. I suppose that with the DXT, I am prevented from doing that.
Middle click DXT (not very accurate test) 90-100 grams. ( does not sound like the same switch as the left and right click)
Inside most mice with a scroll wheel, the switch for middle-click is not under the wheel, but on the side of it, under one end of the wheel's axis. The other end of the axis is fixed.
Because the axis is pressed in the middle, there is lever action that increases the force needed to actuate the switch at the expense of travel.
Some mice -- especially the tilt-wheel mice -- have the scroll wheel in a cradle, with the switch in front of or behind the cradle.