I am putting in a new flue for a Pot Belly stove. The ceiling through which the flue goes slopes at 12.5º. I want to cut a hole in a metal plate that will be fixed to the ceiling, and through which the flue passes. So, a vertical circular flue, radius R, passing through a ceiling sloped at 12.5º to the horizontal. How do I get the shape of the hole? I assume it is going to be an ellipse with short axis A, long axis B, where: A = R B = R/Cos (12.5º) My thinking is to: 1. Generate a circle of Radius R in Photoshop. 2. Transform (stretch) the circle horizontally by 1/Cos(12.5º). It becomes an ellipse. 3. Print the result, lay it on top of the sheet metal, punch a few holes as markers, then cut it out. Is the mathematical part of the above correct?