My understanding of coolant as follows:
The coolant (green or red stuff) contains ethylene glycol which is primary used to raise the boiling point of water above 100 degree celsius and also to stop the water & coolant mixture from freezing in sub 0 temps (not needed as we always stinking hot in KL!). In addition to this it helps to lubricate the water pumps seals (very important) and prevent corrosion of aluminium & iron engine internals - you'll notice it is a little oily.
However, ethylene glycol (coolant) also reduces the heat absorption of water and thus reduces its ability to transfer heat from the engine to the radiator therefore the more coolant you have the less heat transfer too = higher water & engine temps.
The water wetter is to reduce localised boiling by reducing bubbles in the water and in turn increases water to metal contact for better heat transfer. Air you should remember from chemistry/physics is a very good insulator (i.e. doesn't conduct heat very well).
Hence, if you put the above understanding into practice you should keep coolant mixture low ie less than 30% IMO for optimal effect i.e. good heat transfer of water and slight increase in boiling point of water.
Also as far as radiator caps go they are designed to raise the pressure of the cooling system as a higher pressure equals higher boiling point of water too. BUT too high a pressure (i.e. 1,3bar) stresses the o-rings in your engine leading to premature failure = full rebuild required.
Hence IMO use 0.9bar to be safe or max 1.1bar if you need a little higher boiling point of the water.
I trust the above explains the whole thingee and removes any confusion?!?!