I remembered that chart somewhere, but it is not accurate (especially after you get to lvl 25).
The reason is that the chart gives you the place where you can get 100% of the monster's exp, not where your exp gain is maximum. Afterall, lvl 80 monsters gives more than 2x exp than a lvl 40, so 50% of a lvl 80 is more preferred than 100% of a lvl 40. The general rule of thumb is to kill the highest lvl monsters possible, without sacrificing efficiency.
Edit: Only for clvl > 25, and that no one is doing trist or hell chaos runs for you.