There is not a fixed level of exposure difference. The amount of overexposure as judged by a RAW histogram depends on the metering method, the camera and the light distribution over the scene and dynamic range. This is judged from my experience using Canon and Nikon cameras (1Ds III, 5D series and D800E and the older Canons were not that different in this respect). You can easily have blinkies on the LCD and no clipping of the RAW file. You can have clipping or underexposure using 0EV exposure compensation with matrix metering. Also you have quite some clipping on the LCD and some clipping in RAWdigger and Lightroom will still not show clipping with default parameters and AWB from the camera.
In my experience the best method (unless you are a big fan of using exposure compensation all the time) to simply bracekt your shots and choose the best exposure in Lightroom. This usually would be the most exposure without clipping.
In the case of the picture from Michael on a Canon or Nikon this would be a +1EV exposure compensation and show some underexposure and a +2EV might have a little clipping.
I feel the picture was Michael was still a bit flat. My experience is that opening up the shadows and highlights need to be compensated with not just clarity but with contrast and exposure adjustments (of course).
The image looks natural if you have seen similar storm clouds. You may be right that some contrast and exposure is needed, but the description of the processing is still correct. Most of us have our software on auto levels which makes those adjustments when it first opens the image.
For example on my similar cloud shot which i posted as "Storm Coming" Raw Therapee opens it with auto levels. I clicked neutral to see where the histogram really is. It was compressed in the middle with a long tail to the right. I set clipping from default 0.02 to 0, clicked auto levels to put it back, set vibrance to 10, then saved as 90% jpg. That put a very light banding in the clouds in case some internet scourer tries to blow the clouds up to be the sky in their assignment or brochure.
As you say, bracketing is important to make up for any strangeness in the metering. The habit I have for landscapes is to bracket so that the darkest shot of 3, 1 ev apart is ETTR. That gives me lots of shadow data I can pull from the other brackets if I need it. I also know I can process with no (or very little) noise.
