Measure and mark an area 340 square feet - the shape does not matter as long as it is 340 square feet as this is 1/128th of an acre (43,560 square feet)
Make sure and mark the area so you know where to spray.
TIP: Make your area in a setting similar to where you plan to spray. IE: if you are going to be spraying in tall grass, make the area in tall grass - if you are going to be spraying gravel, make it on gravel.
Using clean water (no pesticide added) spray the area while timing how long it takes to complete. Try to spray it the same as you would in the field.
TIP: Spray the area 3 or more times to get a good average.
TIP: Try to maintain even and repeatable pressure when spraying with sprayer that doesn't have a pressure regulator.
After you have found your average time to spray the area, spray into a container ( preferably one that measures ounces) for the time it took to spray the area. Collect using the same pressure as when spraying.
TIP: Also measure this several times to ensure accuracy and variability.
When you have collected the water (or found the average) this number of ounces collected indicates your "calibrated rate per acre" in gallons per acre and will be used when mixing chemicals and spraying - see example below.