Are you going to just warm stuff in it or are you going to use it primarily to cook with? If your just warming stuff and not running the chimney out of it I don't think you do. If cooking with it and running the chimney through the top I would include it. Someone may be on later to tell me I'm wrong though so see if you get more responses before you start cutting.
Most RFs I've seen have an exhaust on CC and a short one on the VC.
Do you plan to use the VC for cooking space and as a WB?
On my build I have an exhaust on my CC. Then I will have a shorter exhaust on my VC/WB. I plan to use the VC/WB as just that a cooker and a warming box as needed. With ideas from the gurus here I'm on my way to just that. I have a vent from my CC to VC and one from the FB to VC. I can slow smoke or crank it up or leave it closed off for a warmer (WB). Like I said there's multiple gurus on here. CRS has a build that helped me design wise a lot.
If you plan not to install an exhaust on your CC. I do not think you have to figure it into your calc's, as far as exhaust. Not telling you how or what to build "but" think abt the versatility if you had separate exhausts.
I have dreamed for years what type cooker I wanted. Had a tank fall in my lap. All the dreaming stopped. I got serious on design and doing all I could to maximize the whole cooker. These guys are the most unselfish fellas I've ever communicated with. A cooker builders dream come true. Hope to get to share some Q in person with several one day
jm2cw
Overkill at this point...but I agree with forty creek. I have a damper between my main cook chamber/warmer and another between the firebox/warmer as well as smokestacks on both the cook chamber and warmer. I put one more damper between the firebox and main chamber.
I can cook only 1 chamber (either one), both at the same temp or both but at different temps. This allows me complete flexibility to cook meats at different temps or warm up or cook sides. I love the ability to cook for any different situation.
Just my humble opinion but if you plan on running it all as one unit/one temp all the time, I feel your unit may be undersized (my opinion is that your warmer should be used as main cook space only occasionally as a larger main cooking chamber would be better if needed all the time). Also, by making it all one unit, you may quickly regret your decision....I know I would have.
However....it's your smoker and YOU have to decide what you want and need for your unit.