A veteran of many Paul Simon tours, David Morgan (Steely Dan, James Taylor, Bette Midler and countless others) is currently mixing FOH for Simon's new Surprise tour.
Learning to mix from a bad mix location, such as way off-center or under a balcony, is difficult. Here, you need strong faith in your decisions. Hopefully, you had time to listen to the P.A. from a good spot before the show. After adjusting the P.A. to your taste, trust your previous decisions while listening to less-than-perfect audio. This is a good case for carrying SIA Smaart or some other system that will verify the correlation between your mix out of the desk and a test mic placed in a “good” listening position. I also carry a pair of trusted near-field monitors (Tannoys). After delaying them to the P.A., I use them at the least-perceptible volume to fill in what I can't hear from the main system. This is a luxury solution, but I can depend on them in problem venues. And remember, the house sound person is your friend. Accept constructive help if you can't hear the P.A.