Here is how I accurately adjust my combustion air on my hand fed furnace. The previous owners of the furnace installed this hole with their version of a "flap valve" beside the spinner on the ash pan door. I took a piece of metal strapping and cut it to a point that I hold up against a ruler measuring from the floor. This is handy if I ever need to advise the female unit on adjusting the combustion air. I'll say lift the air feed to 10 inches (to run a little hotter) or take it down to 9 1/2 (to run a little cooler) ..... Its nice to be able to adjust the combustion air without guess work involved.. I rarely ever use the spinner valve, maybe just to rev up the fire, other than that it stays closed. Of course this works good when the other variables are consistent such as draft and coal sizing.