Granular
Measuring out and flagging a square footage with a calculated bag count can be used to determine a spreader setting on a given product and its rate, chosen spreader throw width, and ground speed throwing wheel track center to wheel track center. A bag count can be determined by dividing the square footage of the flagged area by 1000, multiplying by the label rate and then dividing by the bag weight, usually 50lbs. Example:

Rate = 4lbs/1000 square feet of given product
Square footage of given area is one acre or 43,560 square feet
(43,560/1000 )(4)/50= 3.5 bags

Once bag count is determined, one then applies with their given machine and product to apply 3.5 bags evenly over the flagged area at their chosen ground speed and throw width with the arc of the throw landing at the center of each pair of wheel tracks.

Free Purdue download of detailed calibration issues and instructions:
https://www.extension.purdue.edu/extmedia/PPP/PPP-46.pdf

Liquid
With water in tank, collect ounces of water sprayed at sprayer pressure for one minute for each spray tip. If there is a drastic difference from tip to tip, it is time to change the tips. If given, for example, 34 ounces then calculate Gallons Per Minute (GPM), which would be 34 ounces divided by 128 ounces in a gallon resulting in .2656.

One then plugs GPM into the Gallons Per Acre GPA equation choosing their preferred ground speed and tip spacing:

GPA = (GPM X 5,940)/(MPH X W)
GPA = (.2656g X 5940)/(3mph X 20")
GPA = (1577.664)/(60)
GPA = 26.29 (or 26.3) gallons per acre at 3mph at machine spray pressure
GPA = 26.3