Indeed, Rw Shlomo Zalman zt”l held that it was 45 degrees Celsius.
Machon Tzomet manufactures their Shabbat devices with this psak in mind, and because of the nature of thermostats, allows for a small error. Therefore, they set their products at a maximum of 40 degrees Celsius (104 degrees F).
I’ll add that the halachah does not say אצבע סולדת בות but rather יד סולדת בו , indicating that any part of the hand that would recoil is considered too hot. Essentially, this criteria determines an upper limit. Once the hand recoils, it’s already too hot, so it’s best to be cautious and set the temperature less than the limit.
One more thing. One cannot compare air temperature to being immersed in water. A human can survive very hot or very cold air temperatures, but not water. At a water temperature of 50-60 degree F, one can lose consciousness within a few hours. On the warmer side of the scale, cells start to die at a water temperature of 106 F.