Few things:
- The longer the kettle sits undisturbed, the narrower the difference becomes, generally.
- It depends where on the grate I measure, because I usually use a single off set coal bed for almost everything I cook. So above coals is different from indirect.
- My MT thermometer reads about 20-25 different from the grates, as a generalization. Others seem to report wildly larger differences which I just have not seen.
- I encourage everyone to use an digital thermometer and document the actual temps in their kitchen oven leaving it on 300 for a couple hours.