>>945798Does it look right? Honestly? Study an object made of the same material under a bright one source light? Like a window or spot light? Remember reflections, and reflected light off of bright surfaces. Its sort of a mistake to be too "scientific" about it. a photo of the objects in your post would likely look different.
If you are really stuck you can, i suppose, take a photo. Adjust levels for a full tonal value. Switch the color mode to lab, then select the L channel, then image-adjustments-posterize it to 8-12 levels, to get a broad idea of the values. But dont use it as a crutch.
A original black and white photo (silver gelatin or whatever) is going to have a more dynamic value range, than a color photo. Plus grayscale digital conversions favor red and green. They murder the blues in an image, cause the blue channel tends to be noisy. This tends to flatten some images. Which is why i recommend LAB as its a bit more accurate, for a simple solution.