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Hi! And welcome to JA...let me see if I can help.
That is actually just a personal call. I have always put coolant and water 50% ratio in entire system. What happens is coolant will still freeze but when you add coolant and water mixed then it will take a lot colder temperature to freeze. In short I professionally have always used mixed in entire system. If it freezes then it will crack the reserviour and cause a leak. For best protection 50% mix water and antifreeze.