Would the recommendation of applying fungicides at 20 gallons per acre and 60 to 80 psi or 30 to 40 gallons per acre and 80 psi be better?

Higher application rates, (e.g. 20 gallons per acre versus 12 gallons per acre) help to increase coverage on the leaf surface. The correct pressure to use is one that gives the desired application rate and a reasonable medium/fine droplet spectrum. Extremely high pressures may cause too small droplets that are prone to drift. On hot days with warm air currents rising from the plant canopy very fine droplets may not get inside the canopy. – Mark Hanna