How about as a first test measure "FRR = (average of all current swaps/85)*100"? This would mean that the 15% fee of Bitfinex is added statically on top of the FRR, as currently if you lend at FRR, you receive only 85% of the average rate in reality.
This would mean the FRR rises the more it gets used and no fancy programming would need to happen (unlike "FRR +/- X%" scenarios, that can also end up cancelling each other out).
Are you proposing including the FRR swaps in the calculation and not just the fixed rate swaps?
I don't see how your calculation will raise the rate due to more FFR swaps being taken. I think the FRR would get a one time boost when the new formula is implemented and then will proceed to drop just as it does now.