Some people will have different opinions , but here mine: I believe your tank should be something like 30 inch dia by 7 ft long.
Is there a common distance from the bottom to cut the FT to CC opening
Looking at the tank from the end appearing a clock, a line from 3 to 9 o'clock would represent your cooking grate in order to give you the most usable sq. inches of cooking area. Then go to pit calculator and get your recomended firebox to cooking chamber opening and working from the bottom up, count out how many sq, inches you need and mark your 2nd line. Now you need to know how much slope you will want on your reverse flow plate, Id say no less than two inches for that 250 gallon tank, so measure up from your bottom line two inches, add the plate thickness of your reverse flow plate and the top of your firebox above that and now you have the height of your opening. I usually add another inch or so for good measure , but the key is not to go any lower than that.
Also... according to the calculator...it says my exhaust should be 38.22 inches. So, I've seen that the exhaust should extend down into the CC almost to rack level, so is this part included in the 38 inches...or is that just what needs to extend up from the top?
I have changed my thinking about how low the exhaust stack should be and now recommend no lower than the top of the tallest piece of meat you would be cooking at the highest rack. Your going to go with 8inch pipe , right? 3 ft should be fine.
One more... the FB air inlets...is it better to have them below the level of the fire (it will be on a rack a couple inches off FB bottom), or at same level...or above?
make your firebox about 20 % larger than what the calculator recomends, going tall. Give your self plenty of room for ashes that will be building up under the fire grate, id say around five inches on this size smoker, and leave your self plenty of height above the fire grate so as the top of your highest flames still have plenty of room and dont hit the top plate of the firebox. Position vent or vents that will direct fresh air under the firegrate, and another one that will deliver fresh air to the flames.
The set up below is what I like the best, the door vent is just above the fire grate, and the handle on the left opens a pie vent on the bottom of the firebox. Also, when ash build up in firebox is too much, I can open the pie vent and let some fall out. But some people dont like the ash falling out and want the ash pan inside the firebox, but then you have to raise the fire grate higher than I have this one.( the ash pan is actually under the trailer deck, and designed to be self cleaning at speeds over 25mph)
I hope this helps some.