Are you working on content for both print and mobile, and want that dark grey to be consistent across different media? That's a fairly complicated business, so you'll need some knowledge of colour management. If an 80% tint of black in a print document is your starting point, first you'll need to establish what targets (FOGRA, GraCol etc) your printers work to. Then, using Photoshop, you'll need to convert that 80% black to sRGB (you'll never account for all the different screens, brightness setting etc, but sRGB is about as good a target as you can get), and use that in your mobile work.
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