Welcome aboard! Are you building an offset smoker or a vertical with the FB underneath it? If it's vertical then the 75% will be more than enough because the calculator is designed for an offset. If you're building an offset you could try a larger stack to draw the heat out of the FB but I would be sure to put a damper on it so you can close it down if you need to.
It is always easier to address the issue now as opposed to trying to fix it once installed. That is a big pit and could be a real pain if it isn’t designed properly.