pit calculator? for a firebox

ok BBQ friends, I just got a hold of a 100 gallon propane tank that is going to make a nice patio size smoker. I was looking at the Feldon;s pit builders calculator and am getting two different answers. If I put in 100 gallons, it tells me my firebox needs to be 20L 20W 20H in inches. but if it put in the size of the tank 30d and 47L it tells me my firebox has to be 22L 22W 22H. I am leaning to go with 22 over 20. but my ? is why would it change the calculations. Also i am looking at 1/4in for the fire box then I would like to insulate it. any other advice
would be helpful. thanks

here is the link.
http://www.feldoncentral.com/bbqcalculator.html

the tag on the tank says 100 gallon cap. I will go with 22x22x22. thanks for the Input.