I use a version of FMags formula (I add more than +1 because I putt more aggressively): 9= 1.1, 10= 1.2, 11= 1.3, 12= 1.45, 13= 1.6, 14= 1.7, 15= 1.8 +/- elevation +1.
Quote: "And you MUST always add the +1 to any elevation reading
because WGT measures every putt down from the centre of the ball to the
green surface. Without this +1 you will have to make adjustments
between long and short putts and all your green speed factors would
generally be too slow."
Quote cont'd: "Also -10% of distance for extreme uphillers, and +1ft for every 20ft of putt into a 30 mph wind and -1ft for every 30ft of putt down a 30mph wind"
Quote cont'd: "The distance is also not the straight line distance to the hole, but the distance the ball would have to travel to get there (so it's greater if the putt is bendy). A rough rule of thumb is to add on half the elevation change you get when you move the marker from the hole to an aim point the same distance away as the hole. Eg, a bendy putt, you move the marker 2 yards to the side of the hole and the elevation changes from +2 to +5, so you are adding an extra +1.5 to the distance of your calculation."
I also add +15-20% for severe downhillers when using this formula. In addition I round up any putt to the next numeral, i.e, if the put is over 10F I round up to 11F. Likewise, if the putt is at exactly 10F or even slightly under, say, 9.8, I'll round up to 10.5F. When I don't do this I come up short, even after adding the +1 in the 1st explanation.