How to cut a mesh at the edge? Is there a way I can cut by continuing an edge?


So I want a cut at the red line, possibility using the knife tool, but I can't use the angle snap, since its not on any particular angle, so is there a way to rotate the camera so its on the same angle as that edge? 

Clarification: Where the red line is, it's just grey space. I want an edge there
If you have a face on the edge perpendicular to the front face, then you can select the face and move the view with ShiftNumPad 1. That will rotate the view so that the edge is alined, then the angle constraint in the knife tool will work.