sex on fertile window?

Renee • Just a girl in love.
Hey guys, so my fertile window according to <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">eve</a> starts tommorow, and I plan on having protected sex with my boyfriend. Would it be okay? 
I know there's always a chance, but I'm always careful. Please give me opinions. Shall I wait?