What level of coolant antifreeze is standard - I am in Chamonix at minus 12 degrees centigrade - what level of coolant antifreeze will there be as normal?

 

You should check with your local dealer. It's not a world wide standard but based on geographical areas. Usually it's plenty to handle what ever temps are normal in your area.

 

50/50 mix should be good down to at least - 30 C degrees.

 

It's to be checked by your Lexus/Toyota garage/dealer (France) and do not worry, it's desgined for -20 / -25°C below it's more uncertain.

I do not do anything when seasons change unless make sure the "gasket" alongside the doors are a bit sprayed with adequate protection to avoid freezing of them and damage under specific weather conditions (rain/humidity and very cold temperature in a short period of time; like when you go to ski, rainy in the valley and frozen at the ski station...).
Otherwise do not do anything else for cold conditions except pouring antifreeze (-25°C) water in the tank for wipers (and try to have this tank always full).

 

As Jim2 said, antifreeze is designed to work in a 50/50 water solution. Any more and you can get gelling and reduced freeze protection but also less high temp protection. If it was 60/40 each way that would not be bad but that is where I would start to think about balancing it out.

They also sell premixed stuff to add to the resevoir if that is all you need.