The Viewing Distance is literally the distance you are away from the final print when you look at it. So in a Gallery that will be a few feet.
Calculating DOF is not exact science as it also depends on your eyesight and how sharp is sharp to you vs. me (we all perceive things slightly different).
Its geared towards print and not 100% magnification on the computer screen.
In that case, you circle of confusion should never be larger then your pixel size.