Even if you left some water behind and theoretically, it made your mixture 40/60, in Atlanta, you still have nothing to worry about. The 50/50 mixture gives you certain degree of protection. And 40/60 gives you just a bit lesser but still, it has to be damn cold to freeze. Not Atlanta-cold, much colder. Peak antifreeze 40/60 protects to -12F. But I am almost certain that as long as you had your heater on, the water drained. It would be nice experiment to turn heater to "cold", flush the radiator and then turn it to "hot". You will probably see more fluid come out.
I have not done coolant change on my '10 or '11 (I have 2), but every other car I've worked on, had a valve on intake, which would need to be open to release air from cooling system. You would fill new fluid, start the engine and open that valve until no air and only coolant comes out.
On '10 you have overheating light. On '11 there is also a blue light that is on when you start cold and it goes off when engine reaches operating temperature. It is helpful, but I would enjoy having analog temp gauge. I hate the instrument panels new Mazda3 have, ones with digital gauges and speedometer. Time to say goodbye to this brilliant company - too much nonsense.
Next - not Mazda
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