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

Ok, ... so I just set the tonearm to be 100% parallel to the platter.  Then I closed my eyes and started to adjust the VTA up and down.  There were subtle changes in the sound as the folks above described. 

So I kept adjusting the VTA up and down until I found what seemed to be the sweet spot.  The playback had good detail and the bass was taunt.  I then re-checked the VTA with my bubble-level.  Surprise, surprise.  The arm was almost parallel to the platter with the pivot side very slightly raised.  Go figure.  I guess I just like sonic swill.  ;)         

No it is just the previously mentioned vinyl magic. Also, probably due mostly to stylus profile, this varies from cartridge to cartridge. My Benz Glider and Ruby really rewarded being fine tuned for each record. The Koetsu does reward being dialed in, but once that is done doesn't show nearly as much difference between records. Although to be honest it is really more a case of it sounds so doggone good I lose interest in tweaking and just want to sit there and enjoy the music it is making. 

I go with Michael Fremer on this one. SRA 92 degrees as seen with a USB microscope. Otherwise set the arm so that the mounting surface of the cartridge is parallel to the surface of a 180 gram record. You will have to go pretty far ( over 3 degrees) one way or the other to make a difference in sound quality. It is not as critical as it is made out to be although I think people with more expensive line contact cartridges should try to be as accurate as possible. If they spend 10 grand on a cartridge they can plop $300 down on a USB microscope. 

My final settings are done by ear, using a couple of well-known (at least to me) records including the well known flying fish record about which there is an article for using that record to set VTA. There are other records as well: I first heard that Clements/Holland, Norman Blake and Tut Taylor record in a listening session decades ago, and Saurerkraut 'n Solar Energy was a killer then. 
But, using it to set VTA is for me, like a knife edge, and gets closer to the zone where you have to adjust for differences in sound, quicker. 
Once I'm dialed in, I don't change much- if it is a stupidly fat record, I'll take account of it, but I'm not down to adjusting per record. I also found that the need to do so varies by cartridge; that is, certain cartridges will sound better if dialed in more to whatever is happening in the grooves. 

9 out of 10 of us wouldn’t be able to measure SRA accurately even with a USB microscope. For most of us that method would be a waste of time and money, unless maybe one has been trained by an experienced person, like MF or Peter L. This has been shown more than once on various vinyl forums. IMO, the best advice is to set the top of the headshell parallel to the LP surface, then go up or down from there if the initial setting does not yield good tonal balance. As many others have already written.