It's 2x or even 3x focal length for pixel level sharpness with the 36 mpix Nikons.
Back to the topic, I'm about ready to throw my D810 out the window. Although it works amazingly well in my studio on a tripod, in the field, I've been coming back with some of my money shots in the blur. I was on the AF-S NIKKOR 24mm f/1.4G ED this weekend and came back dissatisfied. My copy produces sharpness in the center more so than the edges, and clear motion or vibration blur in the crop when not shot from my tripod, even when shot at 1/400, although most of my shots were around 1/200 to 1/320. I haven't done nearly as much experimentation, but I'm going to start going through the motions like you. My 50mm 1.4 performs very well, handheld, very little if any vibration blur if any at all. I'd say 50% of my shots hand held on the wider lens did not come out well, using essentially the same technique in each shot. It was just really odd, that some came out well, and others were off.
I'll take this sucker home tonight and take some suggestions to see what I can come up with.