There is no exact science to it Dennis,it unfortunately comes down to experience,but,don't give in too quickly wherever you start to squeek as they can start to come in from a hell of a long way away.I have lost count of the times when I have just about made my mind up to move on(could be 20 mins)and just as you get through the gap in the hedge only to see a fox steaming across the next field to the call that had already stopped.You must ring the changes as well,because foxes learn very quickly,and if you do miss one you will have to do something very different to get him back within range,but