I think Block % is just a % from all shares since last found block. This is not conected with reward which is calculated by mined shares for last 5Nd where N is current mining difficulty for bitcoin
Yes since it's not a moving target it's easy to keep track of - simply reset it each time we find a block.
It's also of interest to see how much of the current block, shown at the top of the page, you have mined.