As a reference point, it's good enough - it doesn't take other variables into account, so it'd be a huge +/- anyway.
Question is - when does one know if there's not enough or too much?
My body was in a shock going from a 26" old geo XC to a new geo XC - at first I had to resort to a 40mm rise bar with all spacers under the stem. A year later, the stem is all the way down (cropped the steerer), running a 20mm rise bar and...