Hi Ya_k_up! I'm Cakes and I'll be giving feedback on your submission. Overall this is a solid submission. Boxes have various shape and foreshortening rate. Markmaking is smooth and confident. I'm particularly impressed by the tight hatching on those starting boxes. One mistake that I find important to point out is that throughout your submission, the initial Y of some boxes has angles that are less than 90 degrees. Box 111, 115, 169, 170 (in this box two angles are less than 90 degree so we can only see two faces whereas every box in this challenge should have three faces visible) and 248 are some examples. With that being said, here are some suggestions for future warm-ups.

I suggest pushing box shape to a more extreme level. For example, you can do pizza boxes (where the height is extremely short but super long width & length), you can also vary the thickness of it from standard pizza box to almost paper thin; rod boxes (the height is long but width & length is extremely short); You can also try to rotate the box so that one plane appears as small & slim as possible (see the bottom plane of box 245 here).

I would like to direct you to this diagram which hopefully help you improve on back corners. Basically you will add the back corner of the box before the last corner (a.k.a 3rd corner). All colored lines are ghosting lines that you do to have a general feel about how the lines converge and you don't start connecting dots until you feel all 4 corners are decent enough. When I am planning out the back corner, I always keep diverging mistakes in mind, which can be illustrated in this image. I do so by comparing the relationship between the line that crosses the back corner (blue in the image) with two neighbouring lines (1 & 2 in the image). If blue diverges with either of these two, it means the back corner is in an incorrect position so I will choose another position to plot. It can be tricky because you have to take into account all three sets of converging lines and make sure the back corner you place satisfies all three. By placing the back corner before the 3rd, you basically take one set out of the equation so it is a bit more manageable. Don't be afraid to throw as many dots as possible on the page to figure out the best position for the corner.

That's everything I want to cover in this submission. There is not much feedback to give because you have done a good job overall. If you have any questions, feel free to ask and I will try my best to answer them.