I hit another snag, firebox opening

[sqwib]

I think I may be screwed, heres what I got
My tank is 20" in diameter, now if I half that for the cooking grate that brings me down to 10" now if I half that for the thermal plate, that leaves me with 5" from the bottom.
I need 51.84 sq inches for the firebox to the chamber opening, now since my chamber is round and I have a long tank 54" I am extremely close, but under on my opening, but I cant tell for sure because I am not sure how to determine the opening due to the fact that the lower I go on the opening the smaller the opening is, is there a way to determine this on the pit calculator?
Worse case scenario can I bump the plate up till its about 4" under the cooking grate?

 

[meat hunter]

Heres what I learned on mine. The further away the drip/heat plate is away from the cooking grate, the better, within reason of course. The closer it get's to the food, its going to start cooking it by convection. Mine is about 6 inches away. If I could do mine over again, I would lower the plate a few inches. My smoker runs 225-230. However, when I smoke say a brisket, it is always done faster than what seems to be the norm, and I contribute that to the convection cooking process. Now is this bad? Well my smokes get done sooner, but am I taking advantage of the "low and slow"? I personally do not think I am.

If you could, and this is just my thinking on this and if I'm wrong, by all means, someone jump in and correct me, I would keep your opening at the 5" mark.

 

[sqwib]

YEAH thats what I figured.

I wonder if the plate is welded solidly to the tank wouldn't the entire tank absorb heat from the firebox inlet?
Have you ever probed a piece of meat with 2 probes to see if you were getting more heat from the bottom as opposed to the top?