Complex? You think I actually calculate things using that formula even before the days of internet and smart phones? I just know that -40° is where the lines intersect through real life experience. On the coldest morning I've ever lived through in my life, Illinois winter of '84/85, the morning temp was -40°C/F, per my outdoor liquid alcohol thermometer with both scales on it. The wind chill was -75°F per the morning news, and happens to be right around -60°C per your initial post.
Regardless, that morning was, as our Lord High Executioner @cany observed, ...really cold, although when thinking back on it I usually replace the really with F-ing...
