It shows a value "100", so I'm guessing this is not a percentage . So this must be in 1/256ths? Or, if the maximum value for all-white is "255", 1/255ths?
Wait -- that cannot be right, I just called up a simple graphic (black lines on white background) and it shows "255". Maybe you should look at the "Mean" value, 2 items up? Statistics is one of the few fields of maths in which I'm surprisingly bad, I could not recall the difference between Mean, StdDev, and Median if my life depended on it ...
<g> I tried my blur/blur/blur/and blur even more, but it doesn't lead to anything useful after all.
Since the Mean is "109.88" in your image, I suspect this isn't a percentage either If it is 1/256ths, there still is a lingering question ... Luca wants a grayscale swatch, which is measured as a percentage of [Black], that is, 0% is white and 100% is full black.
So would the value needed be 109.88/256 * 100% = 42.9% [Black] or does Photoshop measure "the other way around", as usual for RGB, and is the value of Black therefore 100 - 42.9 = 57.1%?