OK, now we are getting somewhere.
Best to post all the pertinent information (for instance what kind of tank you are using) up front in your first post.
Now that we know that it is an oil tank, your numbers are still good so far.
The FB-CC opening depends on how you are going to lay out the smoker.
Assuming a sort of "conventional" layout for an oil tank, you can treat the bottom of the tank like a 27" diameter section. A cutout with a radius of 13" and a segment height of 12.5" will give you an area of 252in[sup]2[/sup] which is close enough to the 254in[sup]2[/sup] that the calculator calls for.
If you want to have the cutout match the inside of the tank exactly, you can use a cutout of 13.5" radius and a segment height of 12.5" for an area of 259in[sup]2[/sup], which is still good.
It also helps to post a sketch of what you are thinking of building. Doesn't have to be fancy, but will go a long way in allowing people here to know what you want and be able to offer useful advice. Simple way to do this is to make a hand drawing and take a picture of it with your cell phone and attach it to your post.
I hope this helps.