A 50/50 mix gives us about a -40 C temp reading. If you are looking for -20 F (-28C) a blend of 40% coolant to 60% water should give the blend you are going for.
A 50/50 would be best, XXXXX XXXXX get the slightly better freeze protection. That little extra margin of safety is worth the piece of mind in frigid temps.
Hope that helps
I misunderstood your question.
We don't deal with "pure fluid" here. The only thing I can suggest so you get an accurate reading is to stop by a parts store and pick up an antifreeze tester. For the $5.00 you'll spend at least you'll know where you stand when you start mixing.
I can't give you a mixture offhand that will give you readings you require.