The "T" stat setting will vary for each stove, stove pipe, chimney set up, altitude & weather.
Like a carburetor, each one is different.
Finding the "sweet spot" for your stove may take a few burns, then you can do small adjustments for various heat output & weather conditions.
My sweet spot is just above "2", I'd say 2.1 for a good burn, not to hot not to cold.
Warm days 40° range I set around 1.75 & get long 24 hour + burns (if it's not windy, wind increases my draft & I get hotter faster burns)
I have burned all night on 2.6, & get about 12 hours when around -10°, then reload on lots of hot coals (heat output goes down near the end so I add wood just as the heat outputs starts dropping off on the real cold days)
I can do a coals burn down by raking the coals forward & throwing on 2 splits E/W & burning on high (3 or more) but it's not a long burn, 2 - 3 hours but lots of heat.
I have a king but the basics are similar.
Mine gets black glass on burns below 2, & clean (but stains where it was black) above 2.25. (but this varies by weather, "high pressure/low pressure" & wind)
Like mentioned, the drier the wood, the cleaner, but on low burns, regardless of the wood, my glass is dark on the sides.
Pic is what I call "Fire dragon eyes" When black; burning cool long burns, closed eyes & taking a nap
when lighter colored; hotter burn eyes open, lighter yet, wide awake & stay a foot away or get cooked

The middle area stays pretty clean most all the time.
Pic is a 2.5 burn, hot & awake, but can go hotter if needed.