I apologize, as it seems unclear to you. When I wrote within 3 feet of the hole, … I was attempting to say at any point 3 feet from the hole… it could be 3 feet short, 3 feet past, 3 feet to the right, 3 feet to the left… so it is essentially within the length of the putter from the hole… then you are within 3 feet.
Yes if it good to be 18 inches past as Pelz might recommend. However, that is TOO narrow a margin from 50-60 feet. If you get really good at within 3 feet of the hole, then we can either tighten it up to 2 feet, or 1 foot, … or make it only 3 feet past the hole, then 2 feet, then 1 foot… YOU determine how tight you want the parameters based on your skill.
I can absolutely say that if you make it too tough at first, you will blow the drill off as being impossible… or it will erode your self confidence. While drill are good at building skills, they should develop both technical skills (like distance control) and mental skills (like increasing your confidence)