Open your D800 manual to the section dealing with 'Auto FP High-Speed Sync.'
You can set your D800 to Auto FP.
Under 'Menu' go to 'Custom Setting Menu'
Choose 'e-Bracketing Flash'
Choose 'e-Flash sync speed'
Choose '1/320 (AUTO FP)
Push the menu button a couple of times or punch 'OK' to exit the menu.

At this point you have instructed your flash to go off in Auto FP whenever your camera shutter speed is greater than 1/250 second.
At 1/250 or slower shutter speed the flash will be normal.

In the Auto FP mode the flash will go off up to the maximum shutter speed of 1/8000 second and give you a photo. That shutter speed will freeze a fan blade!

I suggest you set up your camera and flash as described above and then set the camera mode to 'Shutter Priority' and dial in a speed of 1/640 of a second.
Aim the camera at a fast moving object, be it a child or a dog and take the picture.
The flash will fire 3 short flashes to obtain auto focus for the camera and then will fire the flash.
The shutter speed of 1/640 second will stop action of most moving items.If it does not, crank the speed up to 1/1000 second.

What do you get?
Looking at the 'Historgram' where is the 'white mountain' positioned with regards to the left and right side of the frame"
Centered will result in a good pic.
To the left will be darker and to the right will be lighter.
The 'Historgram' will always tell you if the exposure is acceptable or not and I use that as my guide with every shot.

I leave my setting at Auto FP and forget it. The only trade off is the flash range shortens as the shutter speed goes up.
Give this a try.
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