As Scott says, 50/50 has been working for most everyone for several decades
now.  I've never had an engine freeze on me, even though I had no knowledge
of higher concentration requirements, as I guess many others are the same.
  Anti-freeze has gotten improvements over the years (read the label),
including increased temp ranges.
  When I've lived in extremely cold locales, experience taught me that I had
to bring my battery in at night, and plug in a block or dipstick heater.
That is, if I wanted it to turn over in the morning so I could get to work
on time.
  A warm engine block warms the oil and coolant too.  Convection will cause
some flow through the system.  So you'll never need to worry about coolant
reaching anywhere near the outside ambient temps.  If you're vehicle is in
an extremely cold area, and you can't run an extension cord to your heater,
then your pretty screwed.  You may want to do as they did in the old days,
light a small fire under the engine (sterno?) to warm it up for starting.
Sounds a bit much, but extreme conditions call for extreme measures.

Mike B.

"The coolant additive proportion must be at least 40% but not more than 60%
to maintain the antifreeze protection and the cooling efficiency."

Still another place:  "For year round driving, antifreeze is added at the
factory for temperatures down to:  -13 F/-25 C  (USA);  -40 F/ -40 C
(Canada) ."

So, I knew the recommendation was for a higher concentration for Canada, but
I had it too high for both Canada and the U.S., so far as the owner's book
is concerned.  For my money, -13 F isn't adequate for parts of the U.S.,
even Oklahoma, as we had temperatures below that in parts of the state this
week