While 1 cfm/sqft is a guideline for residential work, it's important to run a load calc to be sure you take all factors into consideration like construction type, glass, insulation, exposures, shade trees, any special conditions, etc. Also, another guideline is 400 CFM per recirculated ton for residential A/C.
Example - 3 tons = 1200 SCFM (Standard CFM). Using your ductulator, assume .1" per 100 foot pressure drop. Start with the total cfm (1200) and then read the values for the duct sizes, velocities, round duct, and friction per 100 feet of duct.
Round duct - 16"
Square/rectangular duct - 15 x 13 (will depend on how much room you have; adjust accordingly)
There is a website that can help with residential duct sizing, cfm, velocities, etc. It's: