here is a link.
scroll down the page and enter pipe dia and segment ED
ED is one half of the desired height in the opening you have drawn. So if you know what area you are looking for then maybe enter 2" for the segment height and scroll down a little farther and see what the segment area is. If you are low then change the ED number to a slightly higher number until you get to 1/2 the desired area.
If this does not help, give us the tank dia and length, and the firebox dia and length along with what size stack you are wanting. I just went thru all this when planning my RF. Only thing different for me is my rigs have flat ends. I think there is a dome end calc out there somewhere to get the area of the ends of your tank.
I hope this helps.