Build how you want your characters to advance. Take note to what are the increments in which they increase in stats. These digits should reflect in the monster strength...
Sorry if I can't help too much, but the method I use is based off a set of increments based of character development. Because the variations to these increments are not too drastic, I can easily formulate a "Rate of Growth" for monster difficulty.
Ultimately it is all based off of how quickly your characters become strong.