Good or bad plan to have sex with protection in my fertile window

Kira

Alright so I want to have sex with this guy who I trust however the opportunity to do so lies directly within my fertile window according to this app. Of course we’d be having sex with a condom anyway but since condoms are only like 85% effective factoring in user error shroud i pair the condom with plan b after? I don’t have access to any other form of birth control either. Yeah give me advice please💕💕