i have known people to run with neat ( just anit freeze) with no probs but most put a mix in, book say 40% mix, but i alway put 50% in my fiesta rs and it runs in the middle of the temp gauge near on all year round unless its real hot weather then it creeps up abit if in traffic.
if your book says 7.8L total coolnt, then stick 3.9L anitfreeze in, cant go wrong.
if your not sure if's its strong enough ( which it would at 50%) get a anitfreeze tester.