Orthographic Shifting, vertical edge matching

I have a project where I have to make huge background plates for 2D animation. Rendering sections and piecing them together in a paint app is [the] fastest way to do this with my computer. Horizontal shifts are easy, just enter whole numbers on X axis. Vertical is not as strait forward. Is there a number value I can use to shift the Y position evenly?

The closest I’ve gotten to matching up vertical sections is .664 for one complete pan up (stacking a 2nd rendered frame above the first). The next (3rd) frame up shifts 1.328. This isn’t perfect. each rendered frame (section) is 5400 x 3600 pixels. The total image is about 27,000 x 1800, or larger. I have no control over production planning. A huge single image is what I am required to produce.

Thank you for any advice.