I loosely use the divide method*, if my oven is 12inches (9Quart) across the top, I would use 24 Briquettes, using less on the bottom and more on the top, this would be for baking/ roasting as you require top and bottom heat placement. There are a few charts* you can follow, however the one by Derek Bullock is the most accurate one available, You can download it here *I can't stress enough these are just a guideline, and you will still need to adjust as needed to get the optimal temperature you are wanting.