Start with 40mcg, increase by 10mcg every day until you notice bad sides (headaches, shaky hands) - on the next day lower the dose by 10mcg, that's your clen dosage. Eventually your body will adapt to the dosage, thus you may increase it by 10mcg every week.
Clen can be run in many ways, the most common clen protocols include:
-2 days on, 2 days off
-2 weeks on, 2 weeks off
-8 weeks on (first 6 weeks with ketotifen), 3 weeks off