I went to the shop this morning and had the coolant's resistance to cold measured: the leftovers from the 1:1 mix I had prepared (AC Delco Dex-Cool and distilled water) showed -30° Celsius / -22° Fahrenheit. The actual coolant in the car showed -25° Celsius / -13° Fahrenheit. The difference leads to believe that either old coolant had stayed in the car or not all of the distilled water I had used for flushing came out.

Nevertheless I think the coolant levels should be ok. I certainly will keep watching. A bottle with coolant will stay in the car for the time being...

Thanks for all your input. Much appreciated!

 

Just put some 100% in the tank.