Convert a stated oven temperature between Celsius and Fahrenheit and see an approximate conventional-to-fan starting point.
Why the numbers are rounded
Recipes and ovens use practical settings, not laboratory precision. A mathematical conversion may produce a number that no oven control offers, so the result is rounded sensibly.
Use appliance instructions first
Fan strength, calibration and heating elements vary. The common reduction shown by the tool is not universal. Follow the recipe when it specifies fan or conventional mode and adjust from experience with your oven.