Here's is what i use for my PFPX settings, included are edited PMDG 737-700, -800, -900ER profiles and US fuel policy's for Domestic, ETOPS, and Flag Jet, I have them set up for delta but you can change the names to reflect your airline. For actuate in sim weights and loading set the PFPX adult and child Passenger weights to 190 lbs and infant to 0 lbs and make sure the seating setting within the PMDG options is set to two class



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Note: At the time of upload i have the aircraft seating to reflect delta airlines middle seat block due to Covid-19 so the max seating is has been reduced. 

Max seating for the -700 is 124, -800 is 160, and -900 is 179. Those max are the max for the PMDG two class max passengers, set those numbers any higher in PFPX and you may run into loading discrepancies.