You're confusing the two interpretations of "1.5°C". The calculation you're using is interpreting it as the absolute temperature, relative to Celsius zero (which is not the same as Fahrenheit zero!).
What you actually want to do is find how much Fahrenheit _delta_ is equivalent to a 1.5C delta. Try calculating the Fahrenheit value for 20C, and then for 21.5C, and then subtract.
Or you can look at the equation you're using, where the slope is a clear and constant 9/5. 9/5*1.5 = 2.7
