Lots of simple photo tools, even a text editor if you use an image with consistent density, e.g. clear film, Photoshop probably as well but I never use it, if I measured your graph at 92%. Then in photoshop if you took the RGB value of and I am guessing this is monochrome image, and then expressed it as a percentage it should either be 92% or 83% depending on if you told photoshop that the file was not gamma encoded. 83% is 92% ^ 2.2
Ultimately if the real value is 92% then you want to all your calculations with 92%, all behind the scenes calculations need to be done in a linear colourspace, otherwise they don't work... How you display that to a user is something different entirely. Our perceptual model is not linear (mathematically) at all that is why we have things like the L in photoshop in the first place.
If you post the file that is probably the easiest :-)
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