AF Fine Tune is precarious at best for prime lenses. Add in a zoom, it gets much more complicated unless you typically shoot at nearly the same distances and zoom levels. The following are all rhetorical questions and food for thought.
How do you know your testing and findings were correct?
Do your settings change at different distances or when the color temperature and brightness of the light changes?
You have only one setting for a lens. How does your one setting affect focus at the rest of the zoom range and distance settings? Did it change your focus at infinity anyplace within the range?
For me and a zoom, I'd test at probably at five different zoom settings at three focus distances, three times. That's 45 separate test. And if the results were not consistent, I'd have do much of it or all of it all over.
Then with consistent results I'd have to figure how I could best use the single setting I have.
So for me - I go and get some experience with the gear. If I start to see or feel something is not right then I try and locate the trouble area. Then I'll test. I don't look for solutions before I have a problem.
Oh and your 50mm f/1.8 - it is soft at 1.8 and doesn't really sharpen up till f/2.8. Take the same photos of a subject about 2 feet away at 1.8, 2.0, 2.8 and 4.0, and with a background more than six feet away. I think you'll be amazed at the difference.