Chevy, I agree. The max for each level is:
L1 - 800
L2 - 2,500
L3 - 6,000
L4 - 11,500
L5 - 19,000
L6 - 28,000
L7 - 40,000
L8 - 53,500
L9 - 69,000
L10 - 86,500
L11 - 106,000
L12 - 127,500
L13 - 151,000
L14 - 176,500
L15 - 204,000
L16 onwards - 260,000

I think that if teams do not build their stadia to the max then they should not be allowed to get max possession. For example a level 10 tam with a stadium of only 40,000 should not get 5% even if full. I have a level 34 team that loses 1% if only 258,000 fans but play against teams with only 133,000 capacity who get 5%. I know their income is less but that is not the point.