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).