I have found that in water shallower than 5' deep 18" seems to be a good depth to set a float. I've never caught a crappie with mud on it's belly (like bass will have) so I assume that they mostly stay at least 6" off the bottom so that would mean that they are 54" deep and since crappie feed up I half that and it comes to 27". With the float set 18-20" deep the fish that are suspended even 2 feet off the bottom can see the flies above them.