If you want a simple rule of thumb here it is. Most GI docs will prescribe Imuran based simply on your weight, in terms of lbs and mg. If you weigh 200 lbs you can roughly prescribe 200 mg of Imuran. If you weigh 150 lbs then 150 mg. Of course there are blood tests to check to see if your metabolizing it and to see if your in the therapeutic range but your original question had to do with your intake of Imuran and to see if it really differs with other's intake. My answer has to do with the usual starting point and maintanence dose for Imuran users. It can be tweaked according to blood tests as time goes on.
Hope this helps.