Coolant amount in block/head?

[RS12Turbo]

I just pulled my radiator, and drained the overflow bottle. Anyone know roughly how much coolant is left in the engine? I'm wanting to put a bottle of Redline Water with distilled water back in and mix with 20 or 30 percent coolant, because it has been Africa hot here lately. Would it be easier to just drain the block and start from scratch? Also, what is the least amount of coolant I can use with the water wetter to not worry about it freezing in the winter......or does Redline put anti freezing properties in their water wetter?Thanks