Personally I wouldn't worry about horizon lines or anything for the 250 box challenge. Just treat the third vanishing point like the other two. You've made most of the decisions by the time you've drawn your Y, so the only question is how close you place each point. It's an exaggeration to say there are no wrong answers, but there's such a broad range of right answers that it's more useful to approach it with the attitude of just getting a feel for how your boxes turn out when you place the vanishing points closer or farther away. I.e. you place the vanishing point close it looks one way, you place it far it looks another way and which one is "right" will depend on the specific drawing.

Now obviously you posted this because you aren't happy with how your boxes are looking, but the truth is you're having difficulty making your lines converge to a single point and that's why they look the way they do. This is normal, and part of what the 250 box challenge is supposed to help with, but there is no solution other than practice.

You're on the right track just keep going.