So, I'm working on a scene to illustrate this.

#declare Aspect = image_width/image_height;
#declare Right_Vector = <1, 0, 0>*Aspect;
#declare Screen_Plane = z;
#declare Camera_Loc = <0, 0, 0>;
#declare Screen_Center = <0, 0, 1>;
#declare Right_Vector = <1, 0, 0>*Aspect;     //<---------

#declare direction_length = vlength (Screen_Center - Camera_Loc);

#declare half_angle = degrees(atan2( (0.5 * vlength(Right_Vector)),
direction_length));

This gives 33.69

Multiplying this by two gives the default camera angle of 67.380

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