I would say you're doing well! The back edge may seem mysterious at the moment but now that you're moving on to the 250 Box Challenge, you will be working with vanishing points a lot more often. Dealing with finding these edges with the Y-method and such will become more intuitive with time. After knocking down the boxes during the challenge you will find that when Rough Perspective comes up in your warmups rotation, you will have an easier time with finding that edge. Just keep up your ghosting!