You definitely want to set the tracking force at the playing surface. Tonearm balance geometry generally alters tracking force if it set at a horizontal plane different from that of the record.
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One thing to keep in mind is that with the Classic tonearm, the VTF decreases as the tonearm gets closer to the platter level so you do want to make sure the VTF is measured at the record surface level. The scale you have gets pretty close, but is still a bit too high to give you a precise reading. I purchased a digital scale from ebay (APTP445) that has a relatively big, flat measuring surface and I built a step with a bent credit card to measure the VTF precisely at the record surface level. It did make a difference compared to the type of scale you have. So to answer your question, yes I do measure the VTF level with the playing surface and it is critical to do so, especially if your cartridge is designed to perform optimally withing a narrow range as my Delos. .
There appears to be a belief that VTF doesn't vary across the record and that checking it at one point is all that is necessary. Depending on the table and arm, I've found that this is not necessarily the case. So what do you do? Try to check all across the record and determine an average. At least that's what I've been doing.
Andy, you might try experimenting by measuring the differences in VTF between the gauge and your setup LP to develop a calibration adjustment that you can apply when using the Acoustech gauge on the platter.
Start by figuring out how much higher above the platter the gauge surface is than the LP you're using to set VTF. You could set the gauge on a flat surface and place an LP (or LPs) next to it until the heights are the same. For reference, here are the measurements I've compiled from some posts I've read...
1.00 - 100
1.27 - 120
1.52 - 150
1.77 - 180
2.02 - 200
Then, with the gauge on the platter, set your tonearm and cartridge combination to 2.00g of VTF.
Without adjusting the counterweight, place the LP(s) on the platter equal to the height difference you measured and record (and average) the VTF.
You now have the difference in VTF readings between the gauge sitting on the platter surface and your setup LP.
You'll then have a calibration factor that you can apply to the VTF measurements you get with the gauge on the platter. For example, if the VTF difference you recorded was 0.1g higher (the readings averaged 2.1g) you actually set VTF 0.1g lower to get the VTF you're after.
If you're really motivated, you could record measurements for a few different thicknesses to see if the relationship is linear or curvilinear, but I'm not sure that's necessary.
You can protect the surfaces of the albums by using thin sheets of paper.
What range of variation are people getting? I have a cheapie digital scale I got off eBay and I don't get a variation greater than +/- 0.05g irrespective of where I put the scale: on top of platter or next to it (given my set up, this puts the scale at least .5 in. lower). I believe I can hear differences in this degree of variation. However, in my opinion, accuracy is not essential here. Precision is more important. One has to locate the correct vtf by ear anyways, so once one is around the area one wants it's just a matter of being able to locate the preferred setting again after repeated testing of different vtf's.
Even for someone like me who adjusts vta for every record, taking a scale out for every record seems insane.
Despite my suggestion to develop a calibration factor for the Acoustech, I didn't go to that much trouble to set VTF. Close enough was good enough.
I had (and liked) the Acoustech digital gauge but it failed on me. I went back to using a Shure gauge and it works OK, but I missed the convenience and accuracy of a digital gauge. I bought one that was built like a tank (unlike the more delicate "analog" ready gauges with the thin platforms) and made a platform extension out of a credit card that was the same height as my setup LP. Works great; easy to use and reliable.
For Kiddman: Although not precisely consistent, there's a relationship between record weight and record height (thickness). I use a thick 180g LP as a reference to set up a cartridge and use a 0.5 mm Herbie's Donut platter mat to partially compensate for 120-150g LPs to keep the SRA within a relatively optimal range. I also bought a 0.8 mm Donut mat for very thin LPs (Dyna-Flex) but most often just use the 0.5 mm mat. For 160g LPs I either use the mat or use a bare platter depending on the mastering.
It sounds complicated but it's not. I can flip the Donut mat onto the platter or remove it in a matter of seconds. And it does make enough of an improvement to be worth the cost and effort, though I probably wouldn't bother buying the 0.8 mm mat.
I measured out a stack of cds plus my scale as tall as the platter and mat with a record on. I then took off the platter sand used the cd scale stack to set the VTF. The diff was .07g and I have my ortofon mc-3 turbo tracking at 2.05g and my RP6 is sounding better and better as it breaks in and I dial it in. :)
I use this scale: Pocket Digital Scale APTP445, which can be found on ebay for $12 including shipping and is quite accurate.