My owners handbook says 50% by volume viz:-
Cooling System
The coolant expansion tanks and coolant systems are situated under the bonnet.
Under the left hand bonnet is the engine coolant circuit and under the right
hand bonnet is the low temperature intercooler circuit. Both coolant systems
should be filled to half way up the tank with HT-12 coolant and should be
inspected only when the engine is cool. It is essential that the engine is supplied
with sufficient water/HT-12 mixture at all times. In an engine of this type, HT-12
must be present in the water at all times. HT-12 should be kept at a minimum
of 50% by volume.
