For tournament speed greens.
Add or subtract 1foot to/from distance to hole and using a calculator, divide by 1.3.
This should get you fairly close every time.
Example: 20 foot putt, 3" uphill = 23 feet
23 divided by 1.3 = 17.69
I would hit 18 ' or 17' if this was downhill.
Some form of scale is absolutely essential other wise you are guessing, regardless of the amount of practice you have had.
Reading the break is always going to be a lottery, experience will help you get close but only luck (or perhaps some 3rd party help?) will get that ball to drop consistently.
If you miss a putt by an inch or by 6 feet, it is still recorded as a miss.