I am calibrating my z-spray. I currently has the lavender/purple nozzles. I'm running about 3.25-3.5 mph(pre garmin speedometer). I figured 1 gallon = 1268 sqft - 18 gallon tank = about 0.5 acres. Does this sound right? I'm trying to get everything set for pre emergent next week. Do I need another set of nozzles for pre emergent? Seems like you need higher quantitiy of solution. HELP ME PLEASE!!!