I think that the percentage is a rather informative parameter. When a user sees a flat reward, they isn't tied to any coordinate system and can't understand without calculations is it much or not. The percentage based stake calculation also restricts the distribution when the difficulty is low (after a hard fork, for example). In this case one can get way more than 750% if the reward is flat. Not sure is it good or bad, though, but potentially it can lead to some price fluctuations. So to make it more convenient and non-confusing for a user and still to appeal to the first impression wow factor, there could be the current short average percentage calculation alongside with the percentage cap in the OP, probably in the form of a dynamic widget.