Add new referential: percentage to the average

For now, the high level of new members lower greatly the amount of the UD.

This cause that using the UD for prices is less close to the average than expected...

To be more close to the average, we can use another relative referential %(M/N):

P = x / (M / N) * 100

P = Average Percent
x = Amount
M = Monetary Mass
N = Number of members