Clomid and Cycle Length

So I just ended my ten day cycle of Provera today.  I'm getting ready to start taking my Clomid on days 5-9 of my withdrawal bleeding.  The doctor wants me to come in and have blood work done to test my progesterone level between days 21 and 24 of cycle. How do I know what days those should be? I'm really confused. I don't have normal cycles at all. Any help would be appreciated!