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If you make the cap 10% lower ...
... Then you've almost got an NFL style cap.
As it is the cap is about 120% of the average.  If you make it 10% lower then you are saying that the MAX should be only 10% higher than the average.
So you are basically forcing everyone to be within a range of 90-110.  That's going to be hard to do.... 
P.S.  Learn to sign your name coward