Im in WI and use 50/50, and was good for -18 last week, Yes the best is 70/30 30 being water. But 50/50 is good to like -35 or something like that, it says right on the jug, but that is way outside, and its still in the - temps.
I have the floating ball tester. In the summer I run a 50/50 mix and it stays protected to about -5 degrees. I put a 70/30 mix in it in the winter because my truck runs so cold I don't need the extra cooling any time other than the summer, so the 70/30 mix keeps me protected to -45 degrees. Oh yeah, I run Toyota Red!
This should be adequate for anywhere in the lower 48.
If you are concerned that that is not enough, you can increase the mixture, but also according to industry standards you do not want to exceed 75/25. The problem is that ethylene glycol antifreeze solution does not have a very good heat transfer capacity. so your engine will overheat but the coolant won't get hot and your guage won't read hot since it measures the teperature of the coolant.