Ditto the oven Thermo suggestion. When using my Weber Kettle, coals to one side, meat opposite then set the Thermo near meat--it kind of stands up like a picture frame if you can visualize that. Cheap to buy, and they last and dont seem to get out of whack temp wise. It will be filthy after a cook but after cooling and some elbow grease it cleans up fine.
On that note, you can also drop a stick Thermo in your dome lid holes at the same time. Then just get a feel for normally happens with the kind of heat you normally build for the time of meat you are cooking. A couple times and youll have a feel for your answer--based on the variables you create and that are unqiue (fire build, the outdoor elements, etc.)