Clive - the knot shouldn't move at all providing you've tightened it correctly and don't trim the tags down too far. Short tags tend to stick in the rings, long tags flow through.
Mike - Nice tip about the small shot above the bulk. Can't remember that being on the original packaging for Billy's floats, but a good one!
Chris - I appreciate what you say about float stops, but I find they get stuck or move trying to go through the rings. Plus, there's the additional cost!