My eyes are still great, thankfully
I did rigorous testing with various diopter settings to ensure I had it at the optimum setting for me, and I can nail perfect focus most of the time given a very easy target and a few seconds to focus. It is just really difficult, the difference between in focus and just off focus at 1.2, 1.4 and even 2 is barely perceptible in my D750s viewfinder. And by barely I mean I really have to focus hard on an easy target and focus back and forth a bit to tell any difference. What makes it especially difficult is that fact that I often use it in very low light with subjects that are often moving. Feels more like gambling than shooting. :-/
It is quite apparent that the D750s screen is showing me a much wider dof than the lens is actually giving me. Thinking of trying the Canon S type screen as I remember it being far easier to focus the 50mm 1.2 ais with it. I've read about the DF being much better for manual focus lenses, but I need a camera that can pull more than dedicated stills work. It is also used by my non-photgrapher bandmates for stills and video.
It really is a shame that Nikon offers no screens for such use. Thanks for the reply Bjorn, love your work!