I've never thought of "all but" to mean "very nearly". I've always thought of "all but" to mean "everything except". So in (probably) all the cases where "very nearly" works, so does "everything except". Unfortunately, the reverse is not true. I too thought her "all but about 80% switch" meant "everyone except about 80% switch" or in other words "about 20% switch". I thought it was weird to word it that way.