the overflow should be filled to the level, if the rad needs to overflow it'll prob use up the excess space in the bottle, then, if thats full, it'll just spill out everywhere, but it aint the end of the world. if the rad needs water it'll suck it out the overflow bottle, in which case you'll notice its low (but there may still be water in the bottle) and top it up accordingly, there wouldn't be any need to stick water in the rad because its already taken what its needed from the overflow bottle. hope that makes sense. ;)