My Helm book says 48 to 55 psi differential above manifold. (40-47 psi for non-VTEC engines.)

If you don't have a differential gauge, disconnect the vacuum hose from the FPR & plug it. FPR hose nipple is open to the air, & the hose is plugged so you don't have a vacuum leak into the manifold. Then use a normal fuel pressure gauge.

Running lean will increase combustion temperatures. You'll fail NOx, but get lower CO & HC.

If your fuel pressure is low, you won't necessarily run lean. Your O2 sensor will measure it & ECU will try to compensate by increasing injector duty cycle till the O2 sensor is happy.