I have a system which works very well and is never far out no matter the course or green speed: Green speed Fast - subtract 5%
very fast - subtract 10%
Tourney - " 20%
Tourney 12+ " 25%
then add or subtract 2/3 of elevation eg; 6 inches up hill = 4 inches
6 inches down less 4 inches.
So, if I have a 20 foot putt with 6 inches of up hill slope on very fast greens;
20 ft less 2 ft (10%) = 18 ft putt plus 4 (2/3 of 6 inches) = a putt of 22 feet strength.
This gives me a quick & easy GUIDE/ESTIMATE of how hard to hit the putt.
(on SLOW greens I add 10%)