Take a flat putt...
20 foot = hit 90% on very fast (9) greens i.e hit it as if it was an 18 footer, hit 75% for greens with speed of 11 and 70% for greens with speed of 12. For 13 speed greens, hit around 65%.
For elevations, use same formula but add/deduct a foot per inch.
Our 20 footer with a 6 inch drop on a 12 speed green would therefore be hit at 40% (20 foot x 70% minus 6)
A 50 footer with a 10 inch rise on 9 speed greens would be 50 x 90% plus 10 or 55foot (so using a 100 foot putt meter, hit at 55%)
These are approx. Putts with big borrows I think need more power usually as the ball is travelling further, i.e not in a relatively straight line to the hole.
Hope this helps a bit. I am still working on this but my putting has improved a lot lately.