This doesn't really answer your question, but something I've been doing is as soon as I am done cooking (and validating that I haven't inadvertently undercooked something), is I close the top and bottom vents to snuff out the coals. I have an old Kingsford bag and when I am certain the coals are out, I shake the recently used coals to get off the ash and dump the used charcoal into the old bag. When I have a short cook and for whatever I see that I need a little extra charcoal, I add some of the already used charcoal rather than adding fresh briquets. It's helping me get the most out of each bag of charcoal. Did that method last night, where I cooked a half a chimney of charcoal for my wok grilling, dumped the ready coals onto the rack, spread them out, then added a layer of used charcoal. Stir fry, a steak, a pork chop, flank steak.... great way to conserve charcoal.