I tried using a LUFs meter to help make my audio samples equal in loudness, but some notes are always louder or softer than others. No problem if using only 3 samples for guitar, but I struggle when using all 49 samples (one for each note). Hopefully someone can share a better way than repeated trial & error.
Almost all meters have their own weaknesses. However, when it comes to guitar or piano samples, the attack section is the strongest, so a peak meter will do the trick.
In this case:
Make a copy of your Soundfont and work with it.
In Polyphone, select all the samples in the Samples section and set them all to 80% in the "Normalize" section of the "Toolbox" menu at the top.
They will all be set to roughly equal loudness.