This is a starting point.
Hmm, never used percentages. Always used addition/subtraction in inches plus feel. Feel has come through experience.
Those percentages you provided are off, according to my experience.
A 7ft putt on 7s greens I hit 10. For a ten ft putt I hit 13.5. For a 20ft putt I hit 24.5. As you can see a percentage figure of 120% won't get you to the hole.
On a 10s green 10 for 10 and then a gradual easing off the longer the putt.
On a 13s green any distance up to 7 I hit the the full distance. For a 10ft putt I hit very near 8.5. For a 15 ft putt I hit 11.
The examples I've given are for a level putting surface.
The most common error I see when playing alt games on 13s greens are putts inside 8ft that don't reach the hole. If its 5 up 2 then hit 7, not 4 lol.
Here's a funny thing: Whistler Hole 12, a steep uphill putt to make birdie. Say for example the distance is 10ft but up 8in, that works out at 18ft. 10s green I'll hit 16, 13s green I'll hit 15.5...Go figure.
Same distance but reverse elevation you need a 3ft hit for 10s and no hope for 13s lol. Merion Hole 5 comes to mind.
There is no magic formula, but yes like you said one needs a starting point, and mine is as stated above.