Mark your putter with a piece of tape so that the edge of the tape is 24 inches from the end of the club (from the grip end).
In order for the putt to be called good, all of the ball must be inside the 24 inch mark.
You are not allowed to just call you own putt good.
If a member of the group calls your putt good, the group should mutually agree before you pick up your ball.
Any doubt...measure your putt using your marked putter.
If your putter is not marked then have another player measure your putt.
The Gimme Putt Rule