I don't have a butterfly valve, like the higher end Jambos, rather it's a plate that slides over the stack. To me, it's not effective for large temp changes, it's more like "tweaking" (ten~ish degrees). But that could either be me, or the design of the plate vs. a butterfly valve.
Honestly, the best thing to do it fire 'er up and go get some chicken or ribs and toss a split on and see what it cruises at. Then toss another on. Watch when they burn down to coals what happens to the temp. The thing is... try to keep clean smoke and frankly, you get good food even if the temps go up and down a bit, and you'll learn the pit. Plus you'll get to eat. I have not made bad food yet with this smoker. I've made OK, better and holy ****! But while I was learning, I still made pretty darn good food.
UDS / Jambo Backyard / Red Thermopen