I'll quickly point you to the reminders section found here which stresses that you shouldn't worry about the back corner too much.

As the reminder says, the back corner is likely always going to be a bit off and focusing on it will distract you from other importance concepts like converging your lines as consistently as you can. Whether you draw the rear corner or last won't really fix things, the reason the back corner tends to be so noticeably off is that it's drawn last and every inaccuracy up until that point gets magnified in it. That being said you will always have to draw something last, switching the order you draw in will just move those inaccuracies around.

Ultimately the goal of this challenge isn't to draw perfect boxes, there are tools to help with that (rulers, digital tools etc.). Instead we want you to improve your understanding of 3D space which you've accomplished. Keep doing your best to think of your lines as a set, with mileage you'll be able to keep track of your vanishing points better and become more consistent. Just remember that at the end of the day you're human and if you're freehanding you'll likely make some mistakes, so don't stress about it more than you have to.

Best of luck in lesson 2.