Here is the correct solution - identical to how you should handle displacement maps.

You must subtract the "field value" from the image value. In this case, the field is .5, so I subtract .5. This means that across the image, the mid-gray field yields a numerical value of 0 which means no change across most of the surface.

Only where the value is slightly more than .5 or slightly less than .5 do we get a peak or valley on the surface.

This is the correct way to handle mid-gray field bump maps and displacement maps. Again, we almost never see this problem, so it is safe to ignore 90% of the time. But why take the risk? Also, even if you have a pretty good bump map with nice dynamic range, you are still getting a small difference in the rendered output. If you load a good bump map, zoom in close and render with and without the .5 offset. You will often see a difference. It is usually not a difference you would notice unless you do a direct comparison, but it is there nonetheless.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)