I would pretty much agree with what BT said in the beginning, though my opinion is just experience, backed up with little fact, I'd say the dosage for the average person would be as follows:
low - 50-60mg
average - 70-90mg
high - 90-120mg
very high - 120mg+
Obviously issues such as tolerance, brain chemistry, weight, and health are going to vary those figures, but I think they're a pretty fair generalisation. After recently eating some pills and finding out the level of MDxx, I've been able to compare experiences with previous pills, and concluded that most pills I've eaten were between the 'upper average' and the 'medium high' level. Basically, when a pill comes out that most people are rating 9/10, its most likely to be around the very high range.