It is best, (in my opinion), to spray a large area at your usual pace and swath. Carefully mark the start and stop gallons level on your tank; (50 gallons is easier). Use a large rectangular grass area of about 12,000 sqft. Measure the area carefully.
Then calculate the number of gallons applied per thousand sqft. For instance, 50 divided by 12 equals 4.16 gal per thousand.
Naturally, you need a tank with graduations marked or some way to measure the solution or water applied. And you need a measuring wheel or tape to measure the area sprayed.