430 intercom
Are you definitely playing with the GNS430 squelch for intercom? The 430 squelch only affects com reception. I think you're pobably refering to a Garmin GMA340 audio panel?
If yes to the GMA340, I think the best option is to leave the headset vols on your 10-13.4's on full (adjusting the headset volumes will affect the sidetone you hear on transmit).
Set the pilot and co-pilot squelch knobs (large knob on each side) both fully anti-clockwise to leave the intercom squelch OPEN, and each pilot then adjusts their own intercom volume level (small inner knob) on the 340 to a satisfactory level. Once volume is OK, each pilot then gradually turns their own squelch knob slowly clockwise until their mic just turns OFF. Talking with the mic about 1/4" in front of lips should just lift the intercom squelch without any clipping.
I find it best to do the squelch setting once settled in the cruise after take-off. If you do it on the ground first, the ambient noise level rise with high power will cause early squelch triggering.
Many GMA340 owners don't seem to remember that the co-pilot squelch also sets the rear pax squelch level, but the rear pax volume is independant, by pulling the co-pilot volume knob out before rotating it.
Hope this helps. When operated correctly, the GMA340 is a great intercom. Even better with BOSE headsets!