I'm usually pretty confident within 10 feet, by spending about half hour on the putting green every other day. I average 30 putts per round.
There is no definite right or wrong putting grip. I use a modified style of conventional right-hand-low grip, with extended right index finger pointing to the ball and a detached left hand on the top of the handle as an anchor. It works wonders at least in my case.
Next time you play, make a note of the long putts you make and short ones you miss and think about it.