You can use the calculator to get an approximation for what you might need. Then go outside and follow the method you mentioned and dial it in. That is what I have done so far on the 2 hammocks I made and I had little adjusting needed from the number given.
Dave aka WK2
Cubmaster of Pack 640 - Smyna, TN
Eagle Scout 1992 - Troop 86 Brentwood, TN
Tread Lightly and Leave No Trace