If you use an Android phone there is an app called 'Dutch Oven Calculator' its free and tells you how many briquettes you need on different size pots and what temp they will generate in the pot.
http://download.cnet.com/Dutch-Oven-Calculator-Android/3000-2126_4-75328780.html
http://download.cnet.com/Dutch-Oven-Calculator-Android/3000-2126_4-75328780.html