I'd run about 40%
antifreeze, 60% deionized water, and a bottle of "water wetter".
Tap water contains all kinds of metal salts that contribute to corrosion. Think of it this way; they don't want you to run tap water in your household clothes iron, why would you use it in your cobra.
Water wetter helps eliminate thermonucleatic boiling...i.e. little bubbles that form on metal surfaces like what you see on the bottom of a pot of water on the stove. The result is far better heat conduction. It also contains lubricants and stabalizers that keep corrosion at bay.
If you want to go really nuts, look into Evans coolant. I've used it. It works fine, but I don't like having to deal with (or pay for) it.