Okay, I know this is an old post but here is my formula. Its as simple as this, distance +/- the elevation change, times the green speed modifier, divided by the total distance of the putt meter. Here is the green speed modifiers I use;
7.9 Green Speed = X 1.11
9 Green speed = X .90
10 Green speed = X .82
11 Green speed = X .76
12 Green speed = X .69
13 Green speed = X .64
14 Green speed = X .58
Sounds complicated but it really isn't. Here is an example; (my putter has a 30' scale, and the green speed is 12). Ball is 22 feet from the hole, the elevation change is +4", here is the formula; 22 (feet) + 4 (inches) = (22+4) = 26 (feet). Calculation; 26 X .69 = 17.94 (modified feet distance) divided by the putter meter, 17.94 / 30 = .598. Distance is 59.8% of the 30 foot meter. I could use my 60' meter, it would be 17.94 / 60 = .29. Distance is 29% of the 60 meter.