Wow... that sounds like a kiln, Roak! I bet you could smelt aluminum. (lol)
A really handy tool to quantify temperature changes as you experiment on your stove's flow design, is an infrared thermometer gun like this.
They're around $20 and well worth it. They don't measure accurately on shiny reflective surfaces like your stainless steel flue, but they work great on everything else. If you want to measure your flue farther up, simply spray a small dot of flat black paint on the flue, aim the gun at it, and you'll get good temp readings.