You just have to feel it out. Remember that green speed will effect your putts exponentially.
i.e You can hit a 10 footer 14 feet and that should get it there if it's slightly down hill.
at 20 feet it's going to be ~ 30
and at 30 feet it's going to be ~ 45
the slower the green speed, the less elevation, and also the break of the green comes into play. On most putt's on very slow greens you are better off aiming just outside the cup.....
oh yeah and this helps.....