Although it may seem logical that when turning down the heat level (eg, burn rate) on a stoker, slowing down the convection fan would be a no-brainer.
I don't know what the formula may be, but determining how much heat will be radiated through a metal surface (duct), the temperature difference between the two sides of the metal, the composition of the metal (steel, lead, aluminum, etc), and TIME are all factored in. Let's say the metal radiates 100 BTU per hour per square foot between two air surfaces. If a cubic foot of warm air goes by in 1 second, 1/3600 of the 100 BTU (for that hour) will be removed from that air. If the cubic foot takes its time moving by at 2 seconds, then twice the number of fractional BTUs is lost from that cubic foot of air. The next foot of duct, another loss, and so on all the way down the duct. By moving the air quickly through the ductwork, the fewer btus are lost through the metal (and into the basement) from each cubic foot of heated air. Therefore, more BTUs are delivered to the living space.
When I first got my stove, I figured I should be feeling 'toasty' air coming from the heat duct. With only the 265CFM provided by the Dayton blower that came with the stove, what little air came out of the duct next to my living room chair(remember, it was going to 6 other registers, too!) as barely noticable but, it was a little warmer than room temperature. When I slowed down the burn rate and the Dayton fan, there was virtually no air coming out of the heat vent and hence, little or no BTUs, either.
When I bought this house, it had oil heat. I'd never lived in a house with oil forced air heat, and was absolutely amazed at how hot the air coming out of the vents was compared to natural gas that I'd lived with all my life (yes, I'm a big city boy). Not suprisingly, though, was the 'on' cycle of the oil was quite brief compared to what I remembered from gas. As oil went from $.83/gal to $1.24 in three months that winter ('99-'00) and a friend had switched to gas with free-hookup to the street from the gas company, it was a done-deal as soon as I could find a heating-cooling place to do the job. I'd forgotten about how toasty the oil heat was when I fired up the gas that fall. But as oil continued to rise, I was happy.
The lower BTU output from the gas simply took longer to heat the house than oil (5-8 minute cycle with gas, 2-3 minutes with oil). No different than turning down the stoker. BUT...the blower in the oil furnace put out about the same CFMs as the gas furnace! The bottom line is it's all about the CFMs! Whether you put out a lot of BTUs or not so many BTUs, the circular air cycle-rate is an absolute must. I guess these HVAC people must have known that all along!
While it may seem reasonable to slow down the convection air speed for a lower burn, doing so further reduces the number of BTUs delivered to the living area!