Use whatever mix is required to protect you from freezing during the expected life of the coolant.
If you add the water wetter, you might notice a small drop in operating temperature, but I wouldn't hold my breath.
Steve
98 Contour SE Sport 2.5 Duratec ATX The wifey's car
89 Taurus SHO - 246K miles
94 SHO ATX - 190K
1997 F-150 5.4L ATX - The Workhorse 150K.
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