Thanks for the feedback! Yes I'm going to have to go with the conclusion that the lens is front-focusing a bit. I calibrated it at 500 mm but not at 700 mm --- I guess I should do that. So score another point for mirrorless systems --- they don't need AFMA.

With regards to the DOF calculator, I have to say that I think it is a bit optimistic when it comes to the standards we want for wildlife. Rather than the 2 feet of depth that it says I should have, I'd say I have maybe 2 inches of depth that is truly "sharp" to my liking. Maybe the DOF calculator is more appropriate for landscape photos? With wildlife, if you don't get the eye tack sharp then you pretty much want to throw out the image. And there is so little margin for error. So when I have an owl on a perch like that, I'm always taking a lot of photos just to make sure that at least one of them is sharp.

Here is the original R5 shot at 500 mm:




And here is the original 1DXIII shot at 700 mm: