i have a liquid filled rail mounted fp gauge that doesn't seem accurate to me. At cold start i checked the fp line off and have 45 lbs. After driving it for a while i checked fp again at line off and now its 40 lbs.
What kind of Fuel Pressure Regulator do you have? I have seen variations in regulators before. Get a FPR that doesn't change. That's one problem that will drive your state of tune nuts.
I also don't trust those rail mounted gauges at all. They are too small to start with, then you subject them to direct engine vibration and who knows how much heat under the hood. I recommend a nice Grainger gauge with a hose.