I'm pretty sure once you're at the RZ tent, getting your bracelets, you're "in the venue" and there's no re-entry.
We did Red Zone in Minneapolis. The RZ tent at US Bank opened at 430. We had planned on getting there about 530/6. We wanted to be close and weren't super-concerned about being on the rail. Of course, we ended up a bit behind schedule and got to the tent at about 700. At US Bank, there was a fair distance to walk to the RZ, but it was easy to follow the tape-trail to the zone. So, figure we were in place at 720 or so. At that point, Beck should've gone on in ten minutes, however, I think he went on at about 750 or so. There was loads of space and almost no line at the RZ bar. We ended up, maybe, 8-12 people away from the main "branches" of the thrust stage. Our group of 10 friends all remained mostly together but with plenty of space to dance and bounce around.
All of this is a long way of saying that I was pretty stressed about getting there to settle in and be close and, in reality, there's basically no way you'll have a bad sight line no matter what time you get there. You do your check in thing and don't feel worried about getting there. Just get to the stadium by 700 or so, get in and find a spot that works for you and enjoy the show!