This is specifically designed to work with Nikon Capture NX where you want to use UniWB as your in camera default histogram exposure guide.
Here are the steps as I understand them (Iliah and gollywop please chime in your additions and/or corrections):
1. Create a new file in PS large enough to fill your screen dimensions at 100% at 300ppi, make sure your colour space is Adobe RGB (1998).
2. Fill with a foreground colour of L: 50 (your RGB values will be 118).
3. Save as a JPEG (make sure Embed Color Profile: Adobe RGB (1998) is selected), choose Maximum quality.
4. Open that JPEG in NX, make sure your colour space in NX is set to Adobe RGB (1998), choose F to fill the screen(should be 100% and no more), hit the tab key to hide palettes, Mac people Command+Option+D to hide the Dock.
5. Set up your D300 on a tripod in a darkened room(no reflections on your monitor), aim lens at the centre and square to your monitor so only the screen is seen through the lens, set focus to infinity. Set ƒ-stop to 5.6 to 8 to reduce vignetting and shutter to a least 1/2 sec.
6. Make sure Picture Control is set to "Neutral" - Sharpness, Contrast, Brightness, Saturation, and Hue all to 0.
7. Select custom WB - Pre, d-0 shoot the screen to set WB.
8. Set camera to Raw, now expose a frame until the peak of the histogram is slightly to the right of centre.
9. Download ExifTool from <noindex>
http://owl.phy.queensu.ca/~phil/exiftool/</noindex>
10.
Open NEF with ExifTool and read the WB_GRBG values, they'll be 256 xxx yyy 256, where xxx is your Red value and yyy is your Blue value. Divide each of the R & B values by 256 to get the multipliers (for eg. 1.769, 1.121) or look at the line Color Balance for these.
11. Open this NEF in NX and correct the WB, Set Grey Point using Marquee Sample, it shouldn't deviate too far from 1 for both R and B values
12. Now multiply your corrected WB NEF R value by the R result of step #10, do the same for the B value, it should look pink.
13. Using your D300, take a WB setting from this on screen NEF using the same technique in step 5-6.
14. Expose a frame in Raw and check this NEF with ExifTool, you want to achieve 256 256 256 as the ideal.
15. If you don't get this on the first attempt, try again by slightly adjusting the R and B values in your pink NEF, repeat step 14 until you get as close as you can be to the ideal.
That's it, you now have set your camera to UniWB. Take a reference shot so you can load it into the d-1 to d-4 position in the Shooting Menu -> White Balance -> PRE -> Preset (see manual pg. 142). It'll look a bit green, remember to expose correctly, hey check that now accurate histogram.
