Smoke stack help

As you can see from my pics I changed my exhaust from the top and moved it down and to the end. My question is I need advise on how long to make the stack. The cooking chamber is 36x16 and the fire box is 17X16 and the stack is 3 1/4 inside diameter. I have looked on the calculator and can't figure it out. Any suggestions would be appreciated.

Thanks, Im going out to cut her down today!