I don't have any measurements to add. But I figure that when you're close to the heel, it's the relights that pose the biggest danger to the wood. So I do very quick applications of the flame at that point, and if it doesn't relight right away, I'll empty out ash and check if there's any tobacco left before trying again. I also like to tamp in a manner to pile up the remaining material against the draft hole so it's always got a little protection.