I had noticed something very interesting when talking about green speed. If you are playing with TOURNAMENT greens, you have to hit the ball at least 4 ft less than your distance to the hole. For example: if your ball is 12 ft from the hole, you habe to shot it with a srenght of 8 ft.
If you don´t know how much strenght are you giving to your putt, you have to count how many times you avatar moves while you putting power in you shot. My putter has 25 ft of max. strenght and my avatar moves 3 times if i hit the ball with all the power. Then, you have to calculate you power doing 25:3... and that is something like 8.So when my avatar makes the first move, I know that I´m shootting whit almost 8ft of power.
I hope it helps with your putting, it helped me a lot.