Quick reference for myself, when I'm talking to British people
3 degrees Celsius, 37 degrees Fahrenheit
5 degrees Celsius, 41 degrees Fahrenheit
10 degrees Celsius, 50 degrees Fahrenheit
16 degrees Celsius, 60 degrees Fahrenheit
20 degrees Celsius, 68 degrees Fahrenheit
25 degrees Celsius, 77 degrees Fahrenheit
32 degrees Celsius, 90 degrees Fahrenheit
35 degrees Celsius, 95 degrees Fahrenheit
38 degrees Celsius, 100 degrees Fahrenheit
So I guess double and then add 30 roughly works?