Welcome to the site cclark. Your tank size hits on the calculator but your firebox comes in a little small at 27.4. Personally I would increase to 28" square but I like slightly larger fireboxes.
1/3 volume of your tank is 20724.00 while a 27.4 square firebox is 20570.82. A 28"
square firebox will have a volume of 21952.00.
With the 28" firebox the ff/cc opening needs to be 175.62 Sq.in. A half moon of 21.5" x 10" 3/4" will give you an opening of 181.5 Sq.in. The added room will allow for a grease bridge on the bottom to keep accidental overflow into your firebox.
I would use 1/4" minimum for my R/F plate.
Make sure you leave at least your firebox to cooking chamber opening at the opposite end of the of the R/F plate so no bottle necking occurs.
I would add a secondary drain in the lower chamber for clean out.
I hope this helps.