We moved our gate back this past spring.  I finally gave up figuring and just went with a little over a 100', mostly because that's where the posts worked out nice.  I wouldn't do any less if I could do it over.  Its real nice to be able to get a pickup and an 18 wheeler on the drive, out of the traffic.  This has happened more than once.  Either the 'follow me to the farm' or when locking up at night and the load of grain is leaving.