The PI trimmer is used in order to get the same
AC voltage on both V3 triodes.
1) So the best way to adjust it, is using a tone generator with e.g. 400hz and put a large dummy resistor in your speaker jack and turn up the master volume a little bit. The measure AC to ground at both pin 1 and pin 6.
2) Another procedure is setting the PI trimmer so that pin1 gets 6-8 volts
DC more volts than pin 6.
Pin1 will usually have more DC voltage when set for best tone....
3) Use your ears.....listen. And adjust the trimmer for maximum feel, sustain, harmonics, and feedback. Some people like to turn up the amp dampen the guitar strings with your hand a gently rubbing the strings and liste for max high frequencies and hiss.
PS this subject has been throughly covered on this forum, so use the search function to get more knowledge