x-y edge position openGL in relation to lens angle

Hello all,

I was wondering if anyone knows what the relation is between the lens_angle and the screen boundaries. Specifically, I'm interested in knowing what the farthest position of a single vertex can be before it's out of the view of the camera, rendering with ortho 1.

approximated example : with a lens_angle of 44.5 the vertex is at the left window edge when it's x value is -1.2
I would like to know what this x value needs to be when the lens_angle is at e.g. 60. in order to ultimately be able to dynammically change the lens_angle which influences the x value to keep the vertex at the edge...

hoping this makes sense,

rgds.
B.