hellboy, as i recall, when i used to do it i didn't have to turn up all that much, and i don't think you will either..why don't you just try it at normal level and gradually turn it up? can't go wrong doing it like that.
btw. the Countryman earset mic looks to me like it is sort of a cheek-mic type of arrangement..the mic doesn't look in the fotos like it's in front of the mouth. wonder if i have an infringement lawsuit there????LOL