Also bear in mind that there are two different kind of "dottle". One is generally a wet, goopy mess of unburned tobacco. To me, that is not truly dottle, but it seems to be the closest word we have for it. The other are little chunks of black stuff; they are not fine white ash, but when you rub a chunk between your thumb and finger, they disintegrate into black powder. When I have a really tasty, cool smoke, that is usually what is left behind. To me, that means you have gotten all the flavorful smoke out of the tobacco, without reaching incinerator temperatures.