Bear, there are a couple of tricks you could try to help develop the feel. (Nothing that really qualifies as a rule of thumb, though.)
It is possible to run your cursor along the rolling beads, going with the flow down towards the hole, and see how far you end up to one side of the hole, and then consider aiming for a similar distance on the other side (subject to tinkering).
If the putt breaks more than one way, you can try to total up the pluses and minuses in each direction, and their relative speed, to get to the approximate 'average' break for that putt.
Ultimately you have to choose your line and go with it. There is no magic formula except practice and feel, especially when it comes to making those 5 - 20 footers.
The techniques I mentioned above can be quite effective for lag putting, assuring that when you have a long putt you will leave yourself something highly makable for a par, or a birdie on some holes.