When you use CHDK, you can't use the Exif metadatas, they can't be wrong.
Exif *may* be wrong, but it should be consistently right or wrong. So if you verify that it is correct on your camera in a particular shooting mode, you can rely on it.
Also, there is standard exif and canon "maker note". The "maker note" is more likely to be accurate.
So, is it possible to know the real shutter speed value from a .DNG file? (If not, all I can do is to write it on a paper when I take the photo).
The DNG exif should record the override that value that CHDK set.