Clomid question!

So I had an appointment with my doctor today and I’ll be starting Clomid next cycle. She mentioned doing a blood draw a certain amount of days after getting a positive opk to make sure that I did ovulate. But I can’t remember how long after she said (think it was CD 21 but not sure.) There was so much info coming at me at once and it was hard to grasp it all. So my question is: how many days after positive opk did you go in to have a blood test to make sure you did ovulate? Did you just call the day you get a positive opk and set up an appointment from there? Also any extra info that you would like to share, I’d love to hear! Thanks!! 😊