All I and others are saying is that any formula isn't going to be the last word; therefore a tolerance of something more like 5mm is going to be more realistic, given individual idiosyncrasies.
Only trial and error is going to get you from the ballpark to perfection (and what's perfect depends on many factors, some of which vary); it's not something any formula can do for you.
The formulas are to get you to the starting point of finding your best position. Via trial and error, not laser.
I'm sure lasers are invaluable when it comes to transferring your position from bike to bike, though.