The club ratings are just a guideline - it might mean something compared to other clubs, but don't take it as a max, min, or average. You have to practice with the clubs and balls you use and keep records (unless you have perfect memory!) to really know how each club will perform under different conditions of wind, spin, shot type, and power.
Green speed is usually quite consistent in my experience - YMMV. You do have to account for changes in green elevation over the course of the putt, and they can certainly make a significant difference in length and line. Downhill putts on fast greens (>10) are especially difficult for me (and others, I think). Naturally if you ignore a downhill change in elevation and the putt rolls well past, it will usually be uphill on the way back - which will slow it down.
All I can say is Practice - Practice - Practice - but try to have fun at the same time!