A Quick Perspective Lesson

Working on sketches today, so just a quick lesson I want to share with the beginner muralist, I don't think this person will mind that I share her mistake with everyone to learn from.
She purchased my Arched Stone Window instructional DVD, but I'm guessing instead of following all of the instruction, she instead projected an image of the window onto a wall and traced it out. Thats all fine and good, but the lesson is, if you're going to trace the window and the angle of the grout lines, you also need to use the same horizon line and vp as the original. She did not. You can't learn perspective if you ignore it and just copy things. Look carefully where her grout lines are, and where they should be. Her horizon line was lower, so her grout lines looked as if they were going off up into the sky, instead of all meeting at HER horizon line/vp.
This gave the window the appearance of being slanted. Some rules you just have to follow. You need to establish your horizon line and vanishing point first.