I think the Key is like Greg said, develop a pendulum putting stroke. My understanding is that is why the pros are so good, they know the speed of the green, factor in whether it is up hill or down, the grain etc and have a good idea how far their back swing and follow through should be. The key is to have a consistent repeatable tempo to your stroke. Then an idea how far the ball will go if you take the club back 3 inches ... or a foot.

Bob