Hi Chris - basic rule of thumb is follow the same recipe you would use in the home oven.

For example - if the meat should be cooked for 20 mins per 500g @ 200 degrees in the home oven, you can follow that same time frame and temperature in the camp oven.

Derek's heatbead chart for cast iron camp ovens is a great place to work out how many you need, top and bottom  http://www.aussiecampovencook.com/charcoalheatchart.htm

My carbon steel camp ovens are 'thinner skinned' than cast iron so I use a few less beads, but I add 10 - 15 minutes more to the cooking time to compensate.

Use one of these thermometers to get your camp oven up to temperature before you put your meat in
They work really well in wood stoves, gas stoves, electric stoves and camp ovens/camp oven mates.
http://www.aussiecampovenforum.com/cgi-bin/yabb2/YaBB.pl?num=1348733311/0

As for your roast -
I prefer the 'less is best' approach for a standing rib roast... a simple rub with oil and a sprinkle of salt & pepper.
Others like to rub plenty of seasoning on or marinate ahead of time.

For 1.3kg,  I would start cooking it about 230 degrees for about 20 minutes, then take some heat away and lower the camp oven to about 180 degrees for about an hour, then check and decide how much longer you want to cook it for - depending on if you like it rare/well done etc.

Remember, each time you lift the lid on the camp oven you are lowering the internal temperature - add a couple of minutes to the time to cover bringing the oven back up to cooking temperature.
Example: you checked the roast and decide to give it another 30 minutes... but it will take 5 minutes for the camp oven to come back up to 180 degrees, so you need to allow an extra 35 minutes.

It is best to err on the side of caution and have your meal an hour late rather than rush things by being too hot.  Long and slow will give you a nice bit of meat - high and fast for too long gives your dog a nice meal that night.

Measure the internal temperature (use a meat thermometer in the thickest part of the meat, away from the bones) to help guage the 'doneness'.  I believe 70 degrees is medium (shrug)  OR -

Go with the old fashioned way - Poke a skewer into the meaty centre and if the juice runs clear it is cooked, if it is blood stained it needs a bit longer.   This method has never failed me for beef, lamb, chicken, roo or rabbit (and some other meats I would rather forget).  Never poisoned anyone so far 

REST YOUR MEAT! 
When you take it out of the oven, leave it to one side covered with foil or an upturned bowl for 15 - 30 minutes.  Use this time to make gravy from the juices in the bottom of the camp oven.
It allows the meat to finish cooking in the retained heat and lets the muscle relax as it cools. 
It helps stop all the juices running out of the meat when you cut in to it.

Hope this helps
LG