You have a Nikon D800. It has EXCEPTIONALLY GOOD High-ISO performance, and one of the absolute BEST-performing sensors in all of photography. You need to learn to USE AUTO-ISO. Unlike it was a few years ago, AUTO ISO on the top-performing Nikon bodies is an amazing, amazing thing. What MLeek is telling you above is the way a seasoned shooter approaches EVERY single photo situation: you need to determine what the necessary shutter speed is for a good, sharp shot, based on lens focal length, speed of the action, the direction the action is in relation to the lens axis, and how far away the action is, and how large the subject will be in the final image. You also need to determine the f/stop needed to get the kind of depth of field you want to have.
For example, shooting across a dinner table...with a short zoom lens...say you want enough DOF to avoid having to focus ULTRA-precisely. Set the lens f/stop to f/5.6. Focal length 20mm on FF. Shutter speed of 1/60 second, which is 3x the focal length, so, pretty safe, and a heck of a lot safer than say, 1/15 second. Your D800 using its AUTO ISO setting can elevate the ISO or drop the ISO, instantly, as needed, to make sure that f/160 at f/5.6 is the PERFECT exposure for the lighting, wherever the camera is pointed, whatever the light does.
Until I got a recent, high-level full-frame Nikon pro body, I was not sold on AUTO ISO. Also, Nikon's implementation of AUTO ISO is far better than that found in most other brands and most other camera models. THe thing that has made AUTO ISO truly useful in my opinion is the 13.7 to 14+ EV dynamic range, and the rich color saturation that the pro-level Nikon FX bodies offer at ISO levels far above the base ISO values. So...you might hear people badmouthing AUTO ISO...but pay no attention. Your camera is now MUCH,much better in performance than 95% of all other cameras in the world at high ISO values. You need to leverage that, and if you have problems with overthinking things, and it seems you do, you need to LEVERAGE the fine equipment you have, and start learning more about the cutting edge benefits the newest FX NIkons bring to the table, especially when paired with the fine lenses you have.
Once you decide on the NEEDED shutter speed, and the desired f/stop, the AUTO ISO range that the D800 has is a HUGE advantage. You need to dig into the manual, and learn how to set up the AUTO ISO settings. trust me...once you see first hand what a modern, high-end FX camera can do in AUTO ISO, when set right, you will be amazed.