Congratulations on your new Phantom!
I usually set my tether at about nose height, but it's always good to experiment for what works best for you. A longer tether will allow you more slack to work your way around the tree uninhibited, but from my experience, a higher and longer tether can interfere with my bow string a little. Some guys set their tether low and have a shorter bridge. I'd like to experiment with that myself, but it seems like it would limit my ability to work my way around the tree before I run out of tether slack.
As far as lineman's belt, I keep it comfortably close to the tree. While climbing, I loosen it so I can make the steps comfortably up my sticks, then re-tighten to hang my next stick or my platform.
I always ran a stock bridge on my Mantis and never adjusted the bridge on my Kestrel, although I believe the original owner shortened it just a little. I never actually measured it. I recently received a Cruzr XC, so I'm looking forward to messing with different bridge lengths.
Play around with it at ground level and see what feels good. The best way is by trying different settings and longer sits will reveal what you like and don't like. Have fun with it and be safe.