The minimum recomended concentration for Caltex's Extended Life Coolant is 33% & max is 67% . I m not sure about axact amount of liquid required to be used as coolant in Coure But it shud be around ~5 Liter so to maintain 33% concentration u need ~1.5 Liter of Antifreeze/Coolant so better to drain old plain water, then pour 2 Liter Coolant & rest water(Preferably distelled water).
On athe other hand, I dont think adding coolant will over come that heating problem u have mention. Bcoz a good/healthy Cooling system can serve as good with Plain water but not recomended for Continous use.
Before Putting coolant i will recomend u to get ur cars cooling system thoroughly checked. The basic measeure/actions are to verify Leaks in pipes, Servicing of Radiator if Clogged, Replacment of Radiator cap, checkcking Water Pump(Body) function, Proper Operation of Fan including its speed & propoer mounting with Shrouds etc.
GO through the forum & search with key words u will find several posts & threads concluding such solutions as per recomndations of Sr. Technical Experts of PW.A member @Xulfiqar have replied in deatils on variuos threads to cater such problems so check posts replied by him as well.