OPK testing with Femara question

Carly • ⚜️🐊Louisiana Mama. 🐊⚜️ Trying not to raise an asshole.

OB prescribed 2.5mg for CD 3-9 and told me to call the office once I get a positive OPK so they can do blood work 7 days after the positive to see if I actually ovulated. My question is when should I start testing? Does Femara give false positives like Clomid does?

Any help ladies?