Is the duct calculator you are using online?
Also in your calculation you say 400 cfm per ton, but then say 1600 for a 3 ton unit, did you mean a 4 ton unit? (400 * 4 = 1600?)
Next, you use 1000 fpm, is that the standard or what other factors might influence it?
Lastly, if a 4 ton unit can provide 1600cfm does that mean there can only be 8 6" ducts and after that you would need to have a larger unit to accomidate additional ducts?
Sorry to ask so many questions but this is exactly the type of info Im after