A backwash decision needs both a rate and a duration. Multiplying them estimates water used during a constant-flow cycle; it does not establish that your supply can deliver it.
The arithmetic
6 US gallons per minute × 10 minutes = 60 US gallons. This example assumes that flow remains constant throughout the entered cycle. A separate rinse or other cycle requires its own rate and duration before estimating the full regeneration volume.
Why sixty gallons is not a storage recommendation
The calculation describes cycle demand. It does not size a pressure tank, storage tank, pump, or well. Nominal tank capacity and usable water are different quantities, and available delivery pressure remains part of the question.
What the worksheet should preserve
Record the complete-system model and manual reference, required rate, cycle duration, relevant pressure and temperature conditions, and separately verified drain requirements. If any of these is missing, preserve the unknown instead of filling it with a generic rule.
Your next comparison
Use the optional cycle-volume estimate in the backwash checker. Its output explicitly excludes other cycles and household demand. The methods page defines the arithmetic and its limits.