What target resolution are you wanting to support for devices? Is the hand painted bg supposed to equal 1 full screen in Unity? 3840x2160 is 4k so I assume you are drawing taller to support some vertical parallax or something?
An ortho camera of 10.8 basically makes a full 3840x2160 image render full screen, so a lot of factors come into play when making this decision.
If you overdraw your images at 4k but only want to support 1080p then it's probably more normal to adjust your Spine export to .5 scale instead of changing things in unity.
Opinion #1.. others may have different ones 😉