Key: in Rows - Character Levels
in Columns - Difficulty Level / Act
in Cells - average amount of experience you get from monster killed on this level/act.
in the 2nd (White) Row - average base experience of monster on this level/act 

For example if you're level 55 you should play on Nightmare, Act4 (not considering Average Base Experience).
For calculation purposes it's assumed that on each level/act there is only one monster of every kind that is possible to meet on this level.