I think what the other commenter meant to say is that you should use the gross contributions.
If A contributed €80 and took out €100, while B contributed €100 while taking out £80, the net contributions are
A: -€20
B: +€20
Now if €90 euros (50% of the current contributions) are to be raised for new spending, it would be strange to assume that all of it will come from B because they are the only ones with positive contributions. If we go by proportionality, it would make more sense to increase contributions to
I think what the other commenter meant to say is that you should use the gross contributions.
If A contributed €80 and took out €100, while B contributed €100 while taking out £80, the net contributions are
A: -€20 B: +€20
Now if €90 euros (50% of the current contributions) are to be raised for new spending, it would be strange to assume that all of it will come from B because they are the only ones with positive contributions. If we go by proportionality, it would make more sense to increase contributions to
A: €120 B: €150
ah yeah that makes sense