The printed pen line is the thing you need to see? And it has to be parallel and equidistant from the sight line (inside edge) of the frame?
That's a tough one since normal frames have a bit of wiggle room to accommodate the natural contraction and expansion of the thing being framed, and leather is certainly no different in that regard.
Are the pieces mounted to a secondary substrate, and can you post a picture of the edge of the panels?