Hi Bill It is uncertain if the method that you are using (using Optical Black Data) can yield the information that you want (Read Noise and other Noise Data and overall S/N Ratio). It is better to ask the manufacturer (Nikon) exactly what this data is supposed to represent. The fact that it is outputted to a file does not mean that there has been some intermediate processing that one may not be aware of. Have you sent your analysis to Nikon for comment. I think it is important to do that.
There has been various attempt by different people to get to grips with the Noise Data and S/N. One simple approach was to measure the Pixel VAlue and standard deviation of a Grey Scale Photo taken at different ISO's. I am currently evaluating all the different approaches that have been published. At least if they are consistent with each other then the S/N data should be reasonably close within the margin of error. Depending on one approach may be somewhat problematic as you may have data that is offset for some reason known only to the manufacturer.
Manufacturers dont like to disclose this kind of data (Noise Data) so I am not sure that they will oblige by writing it out as Optical Black Data for anyone to access. This information is Commercial In Confidence, its the kind of stuff which is kept in closed cabinets at some research centers. If not, then you would see all the manufacturers publishing Noise data and S/N for the Imaging Chips and fr their Cameras. The fact that they dont tells its own story. However I believe that the information (Noise Data) is hidden in the Image and there must be a sound technical approach for extracting this data..