I have had good luck with this strategy for my design clients. 90% go with the middle level (What they typically request and get from me on normal project) one has gone for the highest level, and two have gone for the lowest level. You are right about the lowest level being tricky. My lowest level has all design work but not site meetings- I am now telling them that site meetings are available at that level on hourly basis, but that most people prefer to just opt for the mid level to ensure they get best service. If they still opt for lowest level, you have to be sure to actually charge the hourly for anything outside that scope.