Correct fuel pressure for 3.1L

Hello, I have a 1990 firebird with a 3.1L Im need to find out the correct fuel pressure, I get 35 psi when I put a gauage on it. The chiltons book says it should be between 40 -47 and
the Haynes book says it should be between 35 - 40. the car is giving me code 44, lean mixture. It starts fine but has a miss. I change the O2 sensor and the fuel filter. and a tune-up
Thank You,

Terry Gorman
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