Crackling distortion

Because of the geometry of the JWM uni-pivot arm, the VTF increases as the cartridge height goes up, everything else being equal. Similarly the VTF goes does as the cartridge height decreases. With the Shure stylus gauge, it works on balance and has a range of motion. Within that range of motion your arm is actually changing VTF. it is very difficult to get an accurate reading with that gauge and the JWM tonearm. Suggest you get a digital gauge which has a non moving platform and then measure your vertical tracking force at exactly the same level as your record.

The tonearm connector (Lemm) must be inserted into the junction box in order to get a correct VTF reading.  Otherwise the unipivot arm will have external forces on it when you try to measure the VTF.
I suggest the following:
1. Have your turntable in the state like you are ready to play a record.  If you twist the wire, whatever, then put a twist on it.  I personally have no twist on my wire and have the Anti-Skate set to the absolute minimum.  One donut at the top of the vertical bar, with the fishing line on the top, and no donuts on the horizontal bar.
2. Build up a platform on the side of the platter next to the tonearm.  I actually use the box that the digital gauge came in and DVDs under the box to get the platform of the digital gauge EXACTLY the same height as a record on the platter.
3. Put a record on the platter, a flat one, make sure it is flat.  Use the center record weight and put it on the box where the digital gauge is, in case you have to extend the platform out pretty far to the cartridge.  The weight on the box, ensures you are not going to tip everything over.
4. Zero the gauge, line it up.  When you put the cartridge over the platform, since there is a magnet in your cartridge, with the magnetic force you might see a negative weight.  That is why it is important to turn on the gauge and zero it before putting the cartridge over the top of the platform.
5. Lower the cartridge with the stylus exactly in the center of the dot on the digital gauge.  
6. do it more than once, which will kind of give you an average reading.
7.  If your readings are off by a lot, then something is moving, or you don't have the stylus exactly in the center of the dot.  Keep trying.
8. Move the whole setup off to the side when you are adjusting the weight, you don't want your stylus hitting the gauge.
9. Make sure you tonearm is not resting on the armrest when you are doing the weighing.  Make sure it is not even touching the arm wrest.  
10.  Use the Allen wrench, and the back weight, which screws into the arm to fine tune the VTF.
11.  To see the effect of height, after you everything dialed in, add a couple of more dvds under your platform and weigh it again, and you will see the VTF go up, probably by 0.3 grams or so.

Yes, that also works, since like you said you are compensating for the additional height of the Digital Scale with your VTA.  I have also done it that way in the past.  I, however, don't do it that way anymore, because I find it difficult to compensate exactly with the VTA, it is really eye balling it.  The purpose for 2, is you don't have to make the VTA adjustments, because the platform of your digital scale will be exactly the height of the record.  when you look across the record, from the other side, then you should see the stylus resting on the Digital Scale platform, just like it was on the record.  That is when you know it is perfect.  I believe you can be much more accurate that way, then adjusting VTA to compensate, because it is a little more imprecise trying to eyeball that

I use the Ultimate Analogue test lp Side 2, Track 1 Anti-skating test; 315Hz amplitude sweep to +12dbu (Lateral) the sweep begins clean in both channels. Without anti skate as the sweep increases DB the right channel distorts first. I increase anti skate until both channels distort at the same time. I noticed that this is very cartridge Dependent. My current cartridge requires no donuts on the horizontal bar, my other required 2 donuts. Begin with no donuts and the vertical donut at the highest level. If the right channel distorts first, lower the fishing line and top donut. Keep going all the way to the bottom, if the right channel is still distorting first then add a donut to the horizontal bar, and begin again. BTW, about half way through the sweep you will start hearing the right or left channel distort. If you have too much anti skate the left channel will distort first, then you back off the anti skate. I do everything else first, alignment, VTF, VTA, AZIMUTH, all of it. Then I do the sweep test and adjust the VPI mechanical anti skate until both channels start distorting at the same time. It is very precise, too much and the left channel distorts first. Too little and the right channel distorts first.
I admit this is optimized only for the one part of the record which has the sweep.