This is not a generally accepted method, for you can not trust the consistency of the exposure (how reliable is your camera? You don't know it; my 20D is totally unreliable). Furthermore, even lighting is a true problem.
The accepted method is shooting a step wedge, i.e. you use a single shot and that encompassed a dozen or so stops, then you pick the extremes.
I don't find the link, but you can search for Stouffer wedge T4110.
I had the same thoughts as you (I had a feeling the time intervale of my camera's shutter could not very precise), but they are: this is a series of shots 1EV apart, and the log histograms calculated accordingly. As you can see the camera's shutter is as precise as a Swiss clock, it maaged to allocate each shot (non exposure corrected, non WB) in the same precise position of each f-stop of the linear sensor DR:
Anyway, in the way I am planning to calculate the SNR from the pile of RAWs, I don't need the exposure data (the EXIFF info I mean), I don't even need the shots to be exacly 1EV apart of to be uniformly apart, since I am going to calculate the exposure myself, by analysing the levels found in the RAW file. This assumes sensor linearity, which cannot be 100% true as we approach the deepest shadows.
I did not understand this: "The accepted method is shooting a step wedge, i.e. you use a single shot and that encompassed a dozen or so stops, then you pick the extremes.", could you explain a bit?