So I have been looking at my fuel pressure as well...
Full blown fuel return kit, with a fuel pressure regulator, and pressure gauge. No vacuum line connected to the regulator from the manifold. The car is still stock other than the fuel return kit, which also has a 14 ga wire from the batter all the way to the fuel pump.
I had set the fuel pressure at 51psi, as per FSM, with the engine all warmed up. I did notice that the fuel pressure will go up as high as 56psi when the engine is cold, and it does take 10-15 minutes of driving for it to come back down to what I set it to. This is about a 9% fluctuation.
I also noticed that the OEM volt gauge will go from about 14.4, to below 13.6...ish cold vs warm, which is about 6%.
I think it is safe to notch the last 3% of fluctuation on the heat, and the influence it has on the spring in the FPR.
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