mathia what I did - which was trying to keep it simple - was just to figure out the percentage change in speeds and adjust accordingly. If for example you went from green speed 10 to 9, add about 10% to your distance. If you have a 10 foot putt on a speed 9 green and you are used to speed 10, hit it based on how you would hit an 11 footer. If you were going to a faster green, say from speed 10 to speed 12, subtract about 20%. So if you had a 10 footer on a speed 12 green and you are used to speed 10, hit it about the same as you would hit an 8 foot putt.
Once you hit a few, you can adjust to get a more precise distance, and like many other parts of this game, it takes practice and observation.
Hey I may be wrong about simply figuring out the percentage difference in speeds...I sure am no expert and don't want to give out bad info...it's just that this seems to work for me.