According to the Feldon calculator I sent you earlier that is a nice size - 160.5% size differential given your OD's are 20x20x20
If you go to it and put in 60 gals for your CC , then your firebox size dimensions it will tell you all your info and you can play with the numbers and tweak them.Then go to the other circle calculator site below and you can find your exact FB to CC cutout height.JUST REMEMBER EVERYTHING IS ID! As someone has said before on here "everything the smoke touches".The second circle calc site below is a good one/handy for finding square inches of circles also.
http://www.1728.org/circsect.htm
https://www.calculatorsoup.com/calculators/geometry-plane/circle.php
Bill
If you go to it and put in 60 gals for your CC , then your firebox size dimensions it will tell you all your info and you can play with the numbers and tweak them.Then go to the other circle calculator site below and you can find your exact FB to CC cutout height.JUST REMEMBER EVERYTHING IS ID! As someone has said before on here "everything the smoke touches".The second circle calc site below is a good one/handy for finding square inches of circles also.
http://www.1728.org/circsect.htm
https://www.calculatorsoup.com/calculators/geometry-plane/circle.php
Bill