There's a reason that there's two primary dampers placed apart in the ash door. That's to give more even air feed under the grates. See how left/right evenly burned the ash looks in the pan by using the same amount of opening on both primary dampers. You might find that with using one damper for all the air feed there is more unburned bit of coal showing up off on the other side of the pan.
When the coals are burning well you won't need much secondary air, if any. That all depends on how air-tight your stove's loading door, mica windows, barrel seams, and that round cover under the bonnet are. If they leak alot they become more of the secondary air source, so less is needed from the secondary damper. If they seal well then you'll need to use more secondary damper. How the flames react will tell you. You want the secondary gases to burn, but with "lazy" flames, not doing the jitter-bug. That indicates a slower gas flow to give more time for heat to transfer to the room.
After a reload, and the secondary is open more to help burn off the greater amount of volatile gases of a fresh batch, the color is more yellow. When the yellow burns off to blue (blue indicates hotter flames and more complete combustion) then you can close the secondary down just enough to maintain those blues in a lazy dance.
Notice as you add more secondary the dancing gets faster. Add too much secondary air and the air/gas ratio gets too lean to support combustion and the blues disappear. But the highly diluted volatile gas is still there. Same when there isn't enough secondary air. The gas is still there but now it's too rich to ignite.....unless you suddenly open the secondary, or the loading door. Then it can cause the notorious puff-back. Sometimes explosively enough to do damage !!!!! So never bury the fire with fresh coal. Always put on only enough coal at a time that you can still see red glowing coals through each coal layer. Better to fill up the firebed in layers than smoother the fire and have a potentially explosive gas build up in the stove and pipe. And never, ever, choke off all the secondary air so much that the flames disappear.
So far your only using two of the three dampers that give best control for heat output. When you have an MPD installed you'll be adding another 1/3 to your control ability. And, when you have an MPD in the pipe, that roaring that indicates a lot of heat is going up the chimney will be easy to reduce/prevent.
Think of the MPD as a brake on a vehicle rolling downhill. Using it helps maintain a safe speed. Right now your using the primary as the break,..... but that's like just lifting the gas pedal and using the engine as the break. Works ok on a slight down slope, but not good on a steep hill. Then think of cold weather's strong draft as a steep downhill. You need more MPD "braking" to help maintain a safe speed. Warmer weather is more like a slight downhill - you need less MPD "braking".
Because the MPD plate purposely has holes in it, plus they never fit the pipe well enough to completely seal around the edge, you can not shut off the flue gas flow 100%. That's a safety feature so that you can't completely shut off the pipe and have the gases back up into the house.
If you have a good drafting chimney system, you often can run the stove with the MPD closed. That small amount of MPD holes and loose-ish fit that's left can sometimes be all that's needed to maintain a fire, while slowing the flue gas flow to allow more time for heat to transfer to the house, rather than racing up the chimney.
So,... it's primary dampers to feed the fire and set the heat level you want,..... secondary damper to burn the secondary gases,.... and then the MPD to control the heat flow out. Because of so many setup variables, it's a balancing act that is unique to each stove/chimney system. By experimenting you'll find what combination of damper settings (and when), will work best for your situation. So anyone else's settings should only be considered a general guide to find what works best for your setup.
And I strongly agree with Franco's suggestion to install a manometer gauge. They are not expensive and they easily show you what the draft strength is and how to reset the dampers quickly once you've found the "sweet spot" for how your stove heats best using the least amount of coal. What the mano saved me in coal, by being better able to adjust the dampers for best efficiency, paid for itself within two months. And that's just with a range that holds half the amount of coal your 116 can.
Without a mano it's like driving without a speedometer and guessing how fast your going. Luckily the RCMP doesn't give out speeding tickets for stoves each time you hear that roar !
The least expensive mano is the one most of us use. Simple, accurate and very reliable. The best price I've seen in on Amazon.com here, https://www.amazon.com/dp/B009PAN3C8/ref=biss_dp_t_asn
Hope this helps explain why there are normally three damper systems used and why.