Judge how you feel and respond as the guide to whether you use 15mg, more, or less. And just because it's only 3 weeks doesn't mean you don't need a legit PCT. I highly advise running a REAL PCT and at the very least do 20/20/10/10 if you're running nolva, in addition to using a natty test booster. And I'm praying you're planning on running support supplements during those 3 weeks.