Quick putting "length" observations -
Fast greens = 100% -- Very fast greens = 90% --- tournament greens = 75%
Per cent of what? Actual distance plus uphill inches or minus downhill inches gets you a "net distance" per se. That net distance is where you apply the corresponding percentage.
If downhill more than 4 then putt will still be hot so reduce by experience. If uphill more than 2, you might need another foot of putt.
On putts under net 4, I never reduce below 100% - on tour green putts over 15 to about 100, I also reduce putt power 1 foot for every 15 feet - Example: 96 foot putt downhill 4 inches results in 92 feet, then times 75% = 69 feet, but there are six increments of fifteen within that net distance (of 92), so I'd putt 63 in this example.
Works for me - of course, other variables are strong face or tail wind, hogbacks, and severe breaks - you learn by feel. There is no ONE rule.
But this should get you close to pin high contstantly.