Minimum required vision for motor driving is 20/40. So I guess that's minimum for every one. For blind leadership it could be 00/00.
An out-of-towner drove his car into a ditch in a desolated area.
Luckily, a local farmer came to help with his big strong horse named Buddy.
He hitched Buddy up to the car and yelled, "Pull, Nellie, pull!" Buddy didn't move. Then the farmer hollered, "Pull, Buster, pull!" Buddy didn't respond. Once more the farmer commanded, "Pull, Coco, pull!" Nothing.
Then the farmer nonchalantly said, "Pull, Buddy, pull!" And the horse easily dragged the car out of the ditch.
The motorist was most appreciative and very curious. He asked the farmer why he called his horse by the wrong name three times.
The farmer said, "Oh, Buddy is blind and if he thought he was the only one pulling, he wouldn't even try!"