Defiinately set your pressure with the engine running. You can rev the motor a little from the carb and watch if pressure steadies out, the fluctuation on the gauge could be from the needle and seat in the carb opening and closing.
Another way to test is to install a piece of plastic tubing between your fuel press gauge and the fuel inlet tube. Then tape the gauge to your windshield where you can see it and make a few full throttle runs to see where the fuel pressure goes at WOT. Set it for 6 - 7 psi at WOT and you should be set.
E85 racer and E85 carb builder
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68 Camaro, E85 powered 427" small block. 9.96 @ 133 MPH, 1.319 sixty foot on motor. 5.92 eighth @ 116 with a 1.42 sixty breaking beams with back tire on the bottle