Don't know what it says in the manual, but the antifreeze bottles tell you what concentration you need for a given protection. I normally use a 25% mix i.e. 1 part antifreeze to three parts water, which should be enough for most people unless you live in Siberia.
I tend to agree with Pete. It'll say it on the bottle. I would think that 50:50 mix is too much, as this will give you protection down to -37 degrees (as per some Halfords special I've got in the garage). I dont think it'll get that cold, even at the top of Ben Nevis.
