Part of the problem though, is we really don't know what happens once things are over a certain size, simply because few really dangerous ones have hit since people figured all this out. Aside from some sheeting, it's always possible a large enough object would simply distribute the heat internally, and otherwise be unaffected during it's brief passage through the atmosphere. We can guess, we can look at past impacts and estimate, but until we see one coming, and can watch it arrive, and examine it's effects, it's guessing at best.

Which really isn't all that comforting, for some reason...