VTA can only be set by ear, sort of the way lewm describes. Only it doesn't need to take hours. Simply listen a while, a song or two, just enough to get a handle on the overall balance of the sound. Then carefully adjust very slightly up or down. Up if you think the sound could use a bit more air, down if you think a bit too bright and could use more body. Then whichever way you go the sound will either get better or worse. If worse then go the other way. If better then keep going that way until, guess what? It gets worse. Then stop and backtrack a bit.
The trick is very small adjustments. 1mm is not small. 1mm is monster. 0.1mm is big. Small is micro. Small is you will never even see it, not even with a microscope. Small is like a tiny fraction of a turn on most VTA adjusters. Think 1/20 to 1/50 of a turn. Real small. By eyeball small. By ear, different story. That's why, don't look, listen.