Not seen a figure published for the C15 but the B40 with the same nuts as a figure of 28 - 30 ft lbs.
It is hard to apply the torques given the position of 2 of the nuts and on my B40 just tightened firmly and evenly.
Its worth looking at the recent post by ROBT headed "B44 head gasket" and the problem of head gasket being manufacture from too thin material causing the head to bottom on the cylinder spigot before the gasket has been compressed enough to seal the joint..