Coals Top and Bottom: The Counting Rule
The number that matters is simple: take the size of your oven in inches, and that's roughly your total coal count, split unevenly, more on top than bottom.
For a 12-inch oven, that's about 24 to 25 coals total. Split it 16 on the lid, 8 or 9 underneath. For a 10-inch, you're at 19 to 21 total, something like 13 on top, 7 or 8 under. There are charts online that get more precise than this and you can use them, but the ratio is the part to remember: roughly two-thirds on top, one-third on bottom.
Why the split. Heat rises, obviously, but a Dutch oven isn't obeying gravity the way you'd think. The lid is thinner and more exposed to air, so it loses heat faster and needs more coals to hold steady. The bottom is sitting closer to the ground, insulated a little, and it scorches easy if you overdo it. Bread and cobbler and anything with a top crust want that heavy lid heat. A stew or a braise that's mostly liquid can run closer to even, sometimes even bottom-heavy, because you're not trying to brown a top.
Setting it up
Coals on the bottom go in a ring, not a pile in the middle. Space them out like a clock face under the oven's footprint, evenly, none clustered. A pile in the center gives you a scorched bullseye and a raw halo around it. Owen actually worked that out on his own once, said the ring thing like it was obvious, and it kind of was, I'd just read three books that all buried the lede.
Coals on the lid, same idea, ring around the outer edge, not stacked in the middle. If you clump them in the middle of the lid you get a hot spot that scorches whatever's directly underneath it and leaves the edges of your cobbler pale.
Counting is the whole trick
This is where the class gets quiet and I like that. You count out your coals from the chimney with the lid lifter or tongs, actual counting, not eyeballing "about that much." Sixteen means sixteen. It's boring, I know, but boring is the point. Charcoal briquettes burn at a known, steady rate, which is the whole reason we use them and not lump for this. You can count on a briquette in a way you can't count on a chunk of lump charcoal that might be twice the size of the one next to it.
Once your coals are placed, leave it. Don't lift the lid every four minutes to check. Every peek lets heat out and you're basically resetting your own clock. Set a timer, walk away, come back.
Wind changes the math
Can't perfectly eyeball this one either. Utah County wind, especially up near the canyons or out on an open field, will strip heat off your coals fast, and a count that's solid on a still evening runs cold in a 15 mph breeze. What I do: add two or three extra coals total if it's breezy, weighted toward the bottom since that's the ring more exposed to gusts under an open oven. Then I check earlier than I normally would, at the halfway mark instead of waiting. Guessing wrong here is normal. I still guess wrong sometimes.
A caution about tools near coals
Your lid lifter and tongs get hot fast if you set them down near the coal ring, even just resting on the ground next to it. I got a little blister across two fingers years ago grabbing a lifter that had been sitting close to the coals longer than I'd clocked. It wasn't a big deal, a bandaid and an annoyed week of typing, but it was a dumb, avoidable thing. Now I teach with a rule: tools live on the table or hanging off the tripod, never resting on the ground near the fire. Give your gear a spot and keep it there. Gloves, not oven mitts, if you're handling anything actually touching the coals, mitts don't grip and they don't protect against the sideways heat the same way.
My opinion on the panic instinct
People new to this tend to over-coal because they're nervous. More heat feels like more control. It's the opposite. A blasted-hot oven cooks the outside of your food before the inside's caught up, and that's how you get a scorched bottom and a raw middle, which, yes, happened to me on my very first cobbler. Most recipes want moderate, steady heat held for a while, not a blast. If you're unsure, count low and check early rather than count high and hope.
Before next time
Practice counting out 16 and 8 with tongs, no oven involved, just get your hands used to moving that many coals without losing track partway through. It sounds silly. It helps.