32% seems more commonly used here now for preplant and sidedress. For 5-6 years now we've used 12 gpa of carrier, which is mostly made up of 32% UAN, assuming you are only using 1-2 qt/A of herbicide. That gives us about 42# of N. This on conventional till, then it's incorporated a short time later. We split apply our N needs, so we balance it out for what N is needed when, and speed of application. 20 gpa means you'll be stopping for fill more often than if you apply 12 gpa.

If you have a rate controller set for 12 gpa, and then take 12 gallons of UAN and add 2 gallons of pesticide (just a wild example) and leave the controller set for 12 gpa, you are only applying 10 gpa UAN, with the 2 gpa pesticide.

Or, if you do want 12 gpa UAN, then add the 2 gpa pesticide (again, wild example) then set control monitor for 14 gpa.