Most accurate way to adjust for correct vta on vpi classic 1

I've had my Classic 1 for nearly 8 years.

Eyeball the cart level, call it a day.

Maybe bump it up/down if you can actually hear improvement...but what about that thinner used copy you picked up?

Use the best phono stage /cart you can afford, then you will hear what the fuss is all about.

You're going to drive yourself mad trying to figure out what's "right"

If your arm does not allow for VTA on the fly, it will always be different depending on the thickness of the record.  That's just part of analog. If things sound a little dull, you can try raising the back of the arm slightly or change the tracking force a bit.  The tracking force sometimes makes a big difference.  Some people cannot live with the variables present in the analog world - always obsessing about adjustments until it becomes a source of stress instead of enjoyment.  Just goes with the territory. 

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, recorded without effects, 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. 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 do this in both directions.

Thank you people for your input. It is very much appreciated.