Taking Letrozole. When to start using OPK’s?

Brea

Hello! I’m taking Letrozole cd 5-9. Today is cd 8 for me. When should I start using OPK’s? I have the clear blue advanced digital and according to the info on the leaflet, I should start testing today. Should I wait though and start testing after I finish taking the letrozole? Any advice is helpful! Thanks so much!