How do I draw an ellipse for a 7" pipe ping through a wall at 45 degrees ??

I need to make a template from some fibreboard and I want to make a neat job. I have a 7" flue chimney pipe going through the wall at 45 degrees... ive tried various drawing packages, and sure they all produce neat ellipse's... but how do I know when I have the perfect one to draw around. Keeping the width 7" is easy.... but the height ??? please help