The angle of the cut doesn't have to be perfectly perpendicular to the housing. As long as you are cutting low enough to get under the front clear lenses lip, then you'll be fine.
You're looking at the top gray layer of the rear housing, a layer of adhesive sealant, then the lip of the clear front lens, another layer of adhesive sealant (this is what you want to be cutting through), and then the base of the rear piece. Hope that helps some. There's a write up I did in the Articles section for TB headlights, the process is more or less the same, some of the pics may be helpful. Good luck!