aromasin is a aromatase inactivator, but that does not mean much because the body refreshes the aromatase enzyme in ~2 days. but it is still more effective than arimidex and femara. 1 mg ari - 2.5 mg femara - 25 mg aromasin prevent ~60% - 80% - 90% of estrogen formation.
i would suggest to start with 12,5 mg (half a tab) e3d if you would normally take 1 mg arimidex e2d. this is powerful stuff.