"1. If you have a break that goes both ways do you simply enter a negative number for the opposite direction?
2. The break in grids closer to the putter tends to have less effect than the break in grids closer to the hole as the ball slows. If all your heavy break is at one end of the putt do you make any adjustment?"
First, turbo08, there is never a grid line crossing your immediate path to the hole that displays two opposite breaks. So the opposite breaks must be consecutive, and must be read as such, modifying the end break value.
Second, as Weida intelligently points out, you must putt beyond the hole somewhat to precisely avoid what you fear; i.e., the ball sluggishly veering off the predicted putting line as it nears the hole. But be careful! Wildly overshooting the target can set you up for a difficult par, or worse, a bogey, which is maddening after a failed birdie attempt.
Let me add that the excellent stopwatch idea to gauge break speed can be mentally applied by internalizing the pace of the ticking seconds.
Also, there is no doubt in my mind that some of the motivated players using my formula will do better than I in the actual context of putting, due to a better ability to attribute an accurate numerical value to the break, combined with a better ability to ding the putts. Indeed, students will often surpass their teachers, and the disabled old me welcomes that natural development.
All the best!