So my cousin is quite the accomplished Dutch Oven cook. When I was first starting he told me the rule to start with is consider each briquette for 20-25 degrees each. Which always seemed to me to run a little hot. Then I read the thing below that I copied and pasted and if I did that plan below, then the math above, it would be about 500 degrees in my oven figuring 25 degrees per briquette.
The normal formula for Baking is to use twice the number of briquettes as the diameter of the oven with 3/4 of them on the lid. For a 12 inch oven, you would use 24 briquettes with 18 on top and 6 underneath.