Here is the formula to calculate maximum background distances for screen size.
You don’t want the images that appear behind the screen plane to exceed the distance between the eyes. Having the images further than that distance would make your eyes try fuse an image by angling outwards, leading to eyestain and headaches.
To calculate this maximum offset, you need to know the width of the screen you will be viewing the footage. Let’s say it’s a 10ft wide screen.
10′ x 12 (to get a value in inches) = 120″
2.5″ (average inter-occular width) / 120″ = 0.02 or 2%
This means that the offset of the images in the background (behind the screen plane) cannot exceed 2% offset.
If you were to stereo-grade on a smaller monitor for a larger screen: let’s say a monitor that is 24″ wide, you would make sure that your background images would not exceed the 2% offset by calculating:
24″ x 0.02 = 0.48″ which means that the offset on the monitor would have to be within 0.48″ inches.