Just like the old adage to use the palm of your hand in lieu of a gray card, but one stop higher. ... as if gray cards themselves didn't vary enough!
Over many decades of involvement in photography, I have accumulated a variety of products that have '18% grey' areas for metering, I can count 6 different products that I still possess even today..., and just now metering all of these I find only one of them varied from the others by 0.1EV even though the vintage of origin varies across a span of half dozen decades.
Using the Lightroom eyedropper tool on the above shot, the 6 areas generally fall into the range of about 37 - 43, with several of them reading 40-41. Due probably to the usage only for density, and not as widely for NEUTRALITY (neutral grey) it is visible that one of these cards (center) is visibly warmer than others; while it was sold to me as a 'vintage' Kodak-sourced card, it did not come with any packaging to vouch for its authenticity from Kodak. (The four Kodak cards are grouped together, left to right, 3 having original Kodak packaging.), #5 is Douglas, #6 is MacBeth Colorchecker, #7 is EZ Balance and cropped out of the shot above the other 6 samples.
As for the palm of my hand, over the years I have read values that have varied from +1.0EV to +1.5EV compared to grey card, but generally over the past couple of years +1.2 to +1.3EV is the range.
As already stated, the 'thumb' can be pretty wide and there are different thumbs, but the 'thumb' gets you in the ballpark, it serves as a 'sanity check', but is hardly a calibration standard.