I think the question of what number to display is completely distinct from how to visualize it. For the number you can have (N/D-1) as it was before, or exp(-N/D) as it is now, or ln(N/D) as organ suggested. Each has its own pros and cons with respect to the message you want to deliver.

And then you can choose how each possibility should be visualized. If for example you feel that anything above 400% difficulty (which happens for 1.83% of blocks) is an Eternally Damned Cursed Block from Azazel, give it an icon and color to match.