This is embarrassing... It's been years and years since I've done any grilling other than propane. I've gotten functional smoking on SFBs and drums. Using lump and a chimney, just how much lump do I need for a mess of burgers? I figured one chimney would be enough, but quite a bit of the little stuff drops below the charcoal grates, leaving me with not enough to maintain heat. Anyone care to offer some friendly advice?