The best way to view your TC-Helicon device, in relation to the PA, is as a complete mic channel with effects. What that means is that you're doing all of your pre-amplification etc. at the TC-H unit, so when the signal reaches the mixer, it's more like a line level signal than a low level mic signal. Here are the steps to take in order to ensure maximum gain before feedback when using a PA or Stick System like a Bose L1 etc.:
  1. Set the mic gain on the unit as per your manual.
  2. Set the guitar input (if applicable) so that it clips when you play your hardest, then back it off by a couple of db for headroom.
  3. Set the output level (Mix Page) at full.
  4. Set the guitar level (Mix Page) at full if you're using dual mono.
  5. Set the guitar level (Mix Page) to blend with your voice if you're using Stereo.
At the PA:
  1. Flatten ALL eq.
  2. Turn the input trim to zero.
  3. Turn off any internal/external effects for the TC-H device input channels.
  4. While singing/playing at full volume, slowly turn up the input trims until you are peaking at about -3db on each incoming channel strip.
  5. Now set the channel faders at 0.
  6. Slowly turn up the mains while singing/playing at your loudest. 
  7. When you get feedback, turn down the mains slightly. This is your MAX volume for the PA.
  8. Mix the rest of the band in under the vocals.