Mores' law doesn't apply here. It's a kind of bathtub curve.
0:100 is obviously +32F and adding more Prestone brings the temp down... to a point.
50:50 is good to -35F. I've never seen ice crystals the several times I've seen temps at -40 using 50:50 mix so anything more is pretty much overkill unless you're above the 47th parallel or hell just froze over...
60:40, what I call the devils' icebox mix, is good to -65F. It should still give you OK heat. This is pretty close to the bottom of the curve. There are places above the 46th parallel in the middle states that get this cold. I've been to Grand Forks ND and Crookston MN in January and seen -45F but they were closing schools too so it's not business as usual there. New England isn't getting that cold on a regular basis even in Stewartstown and Pittsburg NH or Caribou and Presque Isle ME. We're too close to the North Atlantic. The top of Mt Washington doesn't count. This mix will actually begin to inhibit your heat transfer somewhat but not to the degree you're seeing.
70:30 is actually back up to -55F
And 100% Prestone is slush at +10F so the actual start of freeze is above that... 100% also flows so badly that it's not any good in the summer either.