Open the black frame DNG with RawDigger http://www.rawdigger.com/ , go to "preferences" and disable "subtract black" and then read the values under sigma (σ). The displayed values are the stdev of the black frame which stdev is the definition of noise in engineering.
This way you have the "read noise" or else the "noise floor".
Then find in exif data (exif button in RawDigger) the "White Level" value and subtract from this the exif "Black Level" or better subtract the avg value (rawdigger display) of the inspected channel. This way you have the "Max Level"
Dynamic range is Max Level/Noise floor. This is the so called "engineering DR" for a single pixel.. it's what DxO measures as "Display DR" and what BlackMagic reports as DR.
To make a fair comparison we have to compare at the same output size. So if the DR of 5dIII is for 1920X1080 (2073600 pixels) output and BM is 2432X1366 (3322112 pixels) we have to normalize the later for 1920X1080 output ..
DR(BM1080) = DR(BM) + log2(sqrt(3322112/2073600)) = DR(BM) + 0.34 stops ..
http://www.dxomark.com/index.php/Publications/DxOMark-Insights/Detailed-computation-of-DxOMark-Sensor-normalization
You are correct about the implication of more pixels as shown a few lines before in this message, ... but wrong at interpreting my message.
I mean that if there is no skipping the above normalisation for final size is OK and by resampling/binning 8Mp to 2Mp gives +1 stop DR. But if there exists any line or pixel skipping we have to account for the real number of used pixels.
If for example, due to skipping, only half of the pixels contribute to the final image we have to normalize 4Mp to 2Mp and the differense will be 0.5 stops.
So we agree isn't it ?? More pixels with given DR result in more DR when we refer to the same final size ..