Proper Blind Exposure with a Gray Card

Film responds non-linearly to high and low exposure values- this I knew.

Some Canon Ds habitually underexpose and need 1/3-1/2 stop correction - this I knew.

When using some Nikons I often need to underexpose 1/3 stop - this I knew.

As for whether some manufacturers are using 12% or 18% grey cards- I don't think this will fool reviewers. They will do ISO tests after correcting exposure. Or was there some other point, like bashing Thom?