Digital signal processing for fiber nonlinearity com- pensation is a key enabler for the ever-increasing demand for higher data rates in coherent optical transmissions. A major challenge of existing techniques is that the fiber nonlinear coefficient needs to be scaled properly during compensation in order to reach the achievable signal quality increase. We solve this problem using a low-complexity algorithm adaptively optimizing a metric based on the soft-decision bitwise demodulator used for modern FEC decoders. An analytical model shows that the proposed scheme converges to the optimal scaling factor with a predictable precision, that is validated by numerical results