I just did some more homework on this one and it is a bit worse than I thought. Dave Omak very rightly corrected me on another post and indicated it is best practice to use 150% of the calculator suggestion for CC FB opening and area under the RF plate. The real problem you are seeing is the calculator error that goes with the firebox you specify for the opening. If you correct the firebox to reflect 100% as it relates to the cook chamber, you end up with 112 as the minimum recommended opening; therefore, if you add 50% to make it breathe better, the real number should be around 112 x 1.5 = 168 square inches or a segment height of 8.5 inches on the circle calculator. Here's a link with your numbers for Feldon's
I agree, the options at this point aren't something to be in love with but that will be a big chamber to keep up to temperature in your climate....The alternative may be that you get a forced air system that may not keep up either after you have invested even more work but I don't know what the BBQ Guru guys recommend for chamber/firebox flow restrictions, I do know they offer several CFM fans that may be able to overcome the problem provided you have enough area for a good coal bed. Keep us posted on what you find out and ultimately decide!