oops, the CSV didnt save the formulas. I will redo later, unless someone can verify the ODS or XLS.
From the image, to me it seems the distance-to-target does not come into the calculation, because the reflected rays from the axis-mirror and offset-mirror are parallel,. Because they are parallel, the distance-of-target does not amplify the error.
This setup places the mirrors at both the same angle of rotation (about themself).
what moves is, the offset-mirror not only rotates about itself, but also about the axis-of-rotation (0,0 in the diagram).
or another way to put it , is that it rotates and translates
Imagine a rotation-angle of 45deg. the reflected rays would be straight up at 90 deg.
The center of the reflected ray from the axis-mirror would be at x=0.
The center of the reflected ray from the offset-mirror would be at "cos(45)xOFFSET"
= 0.707 x offsetDistance.
Does this seem correct?