Distance and putt marker control.
Green speeds of 155/165, when facing downhill putts of 1in to 4in, simply subtract the DH elevation from total length of your putt. 170/180s, for DH putts of 1in to 4in, simply subtract the elevation and then subtract an additional 1foot (possibly up to 2F on 180s with 3i/4in). Any putts more than 4in downhill scale more, meaning you need to subtract more for each green speed.
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