Its not too easy to measure the amount of water missing after 1000 sqft. It best to fill it to the very top or to a distinct level mark on the tank. Spray a convenient square area with water, standard speed, using the swath width you plan to use as your standard swath...(say 8 feet) with what you feel is adequate overlap. Nozzles shoud be about 18 inches high or as the nozzle book recommends. Measure the square feet covered carefully. Refill with a measuring bucket; determine the amount used. Find the gallons per 1000 sqft. Adjust pressure for small corrections.
Say you covered 7000 sqft and used 5 gallons. You used 5/7ths of a gallon per thousand sqft...that is about .71 gallons per thousand. 60 gallons covers 85,000 sqft.
Make a fill chart so you can refer to it when filling the tank with your usual product.