Proper dosage will be directly proportional to the amount of aromatizing horomone(s) in the user.
From what I have read, the range for normal users would be anywhere from .25mg-1mg daily. The range would depend on many things (what is being used, the quality of the arimidex (pharm, rc, generic, etc) and the individuals own body)
I would advise starting low and adding little amounts if i felt I was retaining water, or having other SEs