Congratulations on doing 250 Boxes. This is trully a challenge, and one where it's easy to get lost in your drive to finish them that the quality drops near the end for many people. This isn't true for you, you kept your construction careful, and I feel you improved from the first box to the 250th.

That said, there is a certain kind of mistake that you tend to make a lot, and you've noticed it. You underestimate the angle of the back frame, and that ruins the whole construction.

Noticing it is the first step, and it's a good sign that you saw it. What you didn't do, and it's the second part in learning, is work to fix it.

You know that you understimate the angle. Now, when you construct your boxes, overestimate it. You think it's good, give it a bit of a more angle. Try it a few times, and you'll find out exactly how wider you should make it, and it'll become second nature.