Hot take: I checked how much charcoal I burn per brisket and the number made me sit down
I got curious after a cook last Saturday in Fort Worth and actually weighed my charcoal before and after a 14 hour packer. I filled the basket with 11 pounds and when I sifted the ash the next morning there was about 3 pounds of usable lump still sitting there unburned. That means I was tossing roughly 25 percent of every bag straight into the trash, and at $22 a bag that adds up fast. I started banking the leftover coals into a metal pail and reusing them on the next cook, and the flavor did not change at all. Has anyone else weighed their burn and figured out their real cost per pound of brisket cooked?
I ran the same math on my 18 inch WSM and found about 4 pounds of leftover lump after a 12 hour pork butt, so you're not alone there. What helped me was building a smaller fire with the minion method and letting it spread instead of lighting a full chimney, which cut my unburned waste to maybe a pound per cook. Reusing those coals is fine too, just shake the ash off and toss them in the chimney with fresh ones next time.