On "Very Fast" Greens take the total distance and either add or subtract by inches above or below and then divide by 1.2
IE - 20 foot putt with 3 inches above hole would be 20+3 = 23 / 1.2 = 19.16 feet
IE - 14 foot putt 4 inches down would be...14-4 = 10 / 1.2 = 8.33 feet
Works everytime
Tournament Greens = divide by 1.3
Fast Greens = 1.1
Hope this helps.