Economists at Citigroup reckon that a fund ought to start with a baseline of 2% of insured deposits, and then top up that amount with an additional premium to reflect the risk that peripheral countries may leave the euro and that their currencies would then depreciate.
So, what this bank levy does is, essentially, charge an insurance premium for the insurance coverage on offer: and charges it only on those deposits which are not already insured and paying an insurance fee under other schemes.