You need the pressure regulator--just buy it.
And spend the time necessary to calibrate the sprayer. Learn how to walk at a consistent speed.(Or use hand-held GPS). Apply exactly one gallon on concrete. Measure exactly how many sq feet you covered. Repeat until consistent within 10 percent.
Try to apply at about 100 to 110 percent of the label rate--you don't want to apply less than the effective dose.