What juniur said. If you can't make at least 15-20PSI of boost at full load, you most likely have a fuel restriction somewhere. Either a plugged screen in the tank (very likely), or maybe a faulty check valve in the lift pump.
There is no set dimension or number of turns for the fuel screw on the pump. That setting is made on a test bench. It is adjusted until the pump flows "x" amount of fuel at a certain RPM for a certain number of strokes. Lacking a test bench, this adjustment is best made with an accurate PTO dyno, which is what I'd suggest your tractor visit if you can't find your fuel restriction.