Mut zur Lücke: Du bereitest dich auf eine Prüfung vor. Du musst viel lernen und weisst nicht alles (/und du kannst dir nicht alles merken). Aber einen Teil kannst hast du gut gelernt (/kannst du /beherrschst du). Da bist du (dir) sicher. Wenn du das weisst und an der Prüfung teilnimmst (/zur Prüfung gehst), dann hast du (/dann beiweist du) "Mut zur Lücke". You prepare for an exam. You have to learn a lot and you don't know everything (/you can't keep everything). But some parts you learned well. You are confident (/certain) with that. If you go to sit the exam (/go to the test) being aware of that you have "Mut zur Lücke". . . . My niece just asked me to tell you: She once prepared for an oral test. She knew she would not be perfect and was very afraid of that. And then then her teacher didn't choose her to be questioned. - What a luck!

I´m not sure there is an English idom with exact the same meaning ,... An Idiom with a similar meaning is , "less is more" , in german "weniger ist mehr "