I don't think it works, and it makes no logical sense. 

If you want to create a huge fleet, you have to upgrade your fleet capacity which will destroy your economy.  That is fine. Now when that fleet is gone, Why am I still paying for it?

My proposal is that we keep the fleet capacity upgrades, but unlink them from the upkeep cost. The upkeep cost should be linked dynamicaly to your actual fleet supply usage. And I think that the percentage should reach 100% if you use up all 2,000 fleet supply.That means your economy could actually be destroyed as you try to maintain an entire fleet.

Nothing is more annoying than 7 AI whose fleets have all been destroyed but they can't rebuild because they have fully upgraded their fleet capacity. It is counter intuitive.

Say you have 100 fleet supply worth of ships, then you have to pay 5% upkeep

If you have 500 fleet supply used up, then you have to pay 25%

1000 fleet suppy 50%

2000 fleet supply 100%

In my proposal, even if you upgrade to 2,000 fleet cap, but only have 100 fleet supply, you would only pay 5%