What angle should I set the VTA on my VPI turntable?

Just for kicks and giggles, I bought an Elvon "LP Vinyl Record Player Measuring Phono Tonearm VTA/Cartridge Azimuth Ruler."  The kit comes with a super light and super small bubble-level.  Dirt cheap too.

I also rooted around for the box and instruction that my Lyra Kleos cartridge came in.  Lyra recommends that the tonearm be parallel to the record surface and then small adjustments from there.
 
To my surprise, using the cheapy Elvon azimuth ruler and bubble-level, it turns out that my VPI tonearm was slightly out of adjustment.  Azimuth was off by a teeny amount and VTA was off by more than a small amount.  So I adjusted the tonearm and double checked the VTF.  I tried to compensate for the weight of the bubble-level too (about .25 grams).  Although not perfect, I checked my adjustments with the azimuth ruler and then double checked VTF and azimuth again with the bubble-level.

After all is said and done, I think my tonearm is for the most part parallel to the record, but I lifted the back-end by a smidge.  Hardly noticeable on the bubble level.  I thought the lift improved the sound.  It also was a rough justice compensation for the weight of the bubble-level.

After fiddling with the adjustments, I played some favorite LPs and was pleasantly surprised.  There were noticeable improvements in imaging (real sharp), soundstage (better centered and less smeared) and SQ.  All around noticeably better.

I agree that setting SRA should not be used as a tone control. There is just one correct setting. It’s like adjusting the play-back head of a tape recorder. The purists are correct, though, that it could be different for different record labels. A table of those differences has been published.

Here is the method I recommend: Find a recording that people say is good for hearing the differences when you adjust SRA. Do a search here and elsewhere; there are a lot of suggestions out there. If you can’t find one, take a record of a female singer and listen to it over and over ’till you know it very well. I have used Joni Mitchell’s "Blue." Make sure it’s something you like ’cause you’ll be listening to it a lot. Set your tone arm to absolutely level and listen to the recording a few times.

Read all you can about your cartridge and see if reviewers or users have a consensus suggesting tail up or down. Change the angle in that direction a very small amount and listen to the whole record. You are listening for the voice and instruments to "pop" making them more 3 dimensional. It’s subtle, but it’s there with the a stylus having a clear rake. Do that again and again in very small increments. If you do not hear improvement soon go back and stay at level.

If your reading does not come up with a tail up or down consensus. You may have to try this in both directions.
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An alternative method uses the recording Flying Fish HDS 701 if you can, find one in reasonably good shape. If you can do a search to see how to use it.

The "tone control" aspect is once again putting the focus on only the most gross, crude, entry-level aspect of VTA adjustment. The idea of adjusting according to some table of record thicknesses is equally fruitless. Either put in the time and effort to learn to hear what is going on, in which case you will almost certainly agree fine tuning is an art well worth mastering, or don’t in which case set it and forget it.

In any case you already have the one and only thing needed to perfectly fine tune VTA: your ears. 

At best it is always going to be compromise as record lathe cutter head angles can vary between manufacturers, then there's record thickness and finally VTF can impact cantilever angle.