Digital force gauge query

I wouldn't worry about it. Mine is the same way and the variance with mat on/ mat off is pretty consistent with two different turntables. Depending on the thickness of your mat, I would expect your readings are about .10 gram different. It may be anal, but I end up averaging the two readings to get where I want. Of course, there are those who say you should adjust by ear to finalize VTF anyway.

I actually think the mat off reading is probably more accurate for most records, unless most of your records are 180 or 200 gram pressings. Typically, the weighing platform, even on the best of the scales (I use the Canrong scale, also marketed for megabucks more as the Acoustech or Origin Live) is thicker than even a 200 gram pressing, giving you a slightly inaccurate reading compared to what a typical record is going to give you in terms of VTF.

Go for the average and don't sweat it.

Hdm:
I think your advise is spot on. I measured with the mat off as a means to compensate for the thickness of the gauge's measuring platform. I am using the exact same gauge as you, i.e. the Carong. My question is why, scientifically, even logically, should the readings differ merely because the plane of the arm is at a slightly different angle?

Leverage. As the arm is raised the weight shifts toward the counterweight slightly.

1. The entire mass of a tonearm/cartridge is supported at two points, the bearing and the stylus.

2. Downforce (aka, VTF) is an measure of how much force gravity is exerting on the *portion* of the tonearm/cartridge mass that's being supported by the stylus. (The remainder of the tonearm/cartridge mass is still being supported by the bearing.)

3. When you raise/lower your scale (or arm base, as with VTA adjustment) it changes the angle of the arm. This shifts a small percentage of the armtube and it's mass from one side of the bearing support point to the other.

4. More mass on the scale side of the bearing = more weight being supported by the stylus = higher VTF. Less mass on the scale side of the bearing = less weight being supported by the stylus = lower VTF.