Input 37 x 60 as the cylinder size (Cook Chamber = CC). Select the shape of your firebox (FB) and enter a size about a third of your CC (I used 37 x 20 round). Enter the chimney size for a round stack (I used 6"). Enter an estimated intake size (I used 2" x 6"). Hit submit.
FB differential should show 100% (shoot for 105%). Chimney length 38.05". # of Intakes 5.38 (means you'd need over 5 of that size or make larger). Throat size is the FB to CC opening.
As you make changes to the different parameters and re-submit, you'll see how the changes affect the output.