MCS seems to be somewhat out of date, both with that 60% figure and expecting to need 55°C flow temperatures when it’s below freezing. Heatpump technology and system design has evolved considerably; MCS has not.
You may find this topic interesting, which discusses how much to oversize a heatpump to ensure it can provide enough heat to compensate for regular defrosts. It includes data from several manufacturers, as well as real-world measurements from real systems: