Nice work overall! Starting with your cylinders around arbitrary minor axes, you've done a great job in demonstrating a great deal of control over your ellipses, keeping them smooth and consistent, and in aligning them quite well relative to your intended minor axis. You also did a great job of identifying the "true" minor axis at the end to check where your alignment was off, so as to continue improving over the whole set.

There's just one minor issue that I want to point out to you. Right now you're definitely aware of how the opposite ends of your cylinders change in two ways. First off, the farther end is always smaller than the closer end - sometimes by a little, sometimes a lot. We call this the "scale shift". Secondly, the farther end is always wider than the closer end - again, sometimes a little, sometimes a lot. This is called the "degree shift", as it's the degree of the ellipse that changes.

The key thing to keep in mind is that the amount of shift in both categories needs to be consistent with one another, because both of them represent the relative length of the object - or more accurately, they tell us roughly how much farther away one end is from us compared to the other. As it gets farther away, that far end is going to get smaller and wider.

That means that you're never going to end up in a situation where the far end is noticeably smaller but roughly equal in degree (like #50), nor will you end up with the opposite. When this happens, the viewer can often tell that there's something off about them, though they can't always pinpoint what.

Moving onto your cylinders in boxes, these are also coming along nicely. The purpose this exercise serves is actually more about learning how to draw boxes than cylinders, specifically to develop one's ability to draw boxes with opposite faces that are proportionally square. We do this by using the cylinders, and their resulting major lines (the minor axis, the contact points, etc.) similarly to how we use the line extensions in the box challenge. Since these lines will line up with the vanishing points of the box only under the circumstances where the ellipses themselves represent circles in the space defined by those vanishing points, it means that the plane that contains them would also represent a square in 3D space - thusly being proportionally square. As we extend these lines and test them out, then make small changes to how we approach drawing them, we develop the ability to estimate those proportions, often without realizing it.

There is one issue I want to draw your attention to however. I'm noticing that as your boxes get longer (to accommodate longer cylinders), you end up with the lines of the plane on one side converging towards different vanishing points than the lines of the plane on the opposite side. This is pretty normal - it happens because the greater distance between those lines makes it harder to actually think about them when figuring out the intended orientation of the line we're drawing. It always ends up requiring a steeper angle than we expect. Just make sure you keep thinking about that when drawing longer boxes in the future, always think about the vanishing point those lines are meant to converge towards, and think about both ends of these boxes at the same time.

All in all, your work here is solid. I'll go ahead and mark this challenge as complete.