Hello Eric,
You’ve probably sorted your issue with aperture corners by now anyway, but I was stuck on exactly the same point. The thing that I found really helpful was Greg’s video on “Aperture corner development”, quite early on in the Andante stage (about the third lesson). If you haven’t seen it, it may be worth a look. And if you have seen it, it may be worth a closer look 😉

Good luck!

Jutta