I used to mix it at about 68% antifreeze, 32% distilled water to get the maximum protection, both from freezing/boil over, plus I figure antifreeze has better corrosion protection than water, therefore heavier amount of antifreeze should protect against corrosion better.
I no longer do this, and will never do it unless the manual specifies it for the conditions I encounter. Why? Because antifreeze is not nearly as good at heat transfer as water. On a couple of vehicles I had, a 1979 Datsun, and a 1985 S10 Blazer, actually ran noticeably hotter, judging from the vehicle's own temperature gauge, on a 68% solution. Draining, flushing with water, and replacing with 50% solution actually showed lower temps on the temperature gauge.
In today's vehicles, with all the sensors on the engine, etc., I believe it is much more important to have the engine run with the proper mixture of antifreeze and water, which in all vehicles I have had, in the Mid-Atlantic region of the US, is a 50/50 mix. This is what I use.