[This problem is a bit fuzzier than most on this page, just because I don’t know for sure that the cookbook publisher made a translation error. But I hope you’ll still enjoy the problem.]

Recently, I received a wonderful cookbook. In an appendix, it shows a table that converts oven temperatures between Celsius and Fahrenheit. (Side remark: Approximate oven temperatures are actually really simple to convert in your head–just double the number of degrees Celsius to get the number of degrees Fahrenheit. For oven temperatures, this will be within 10 F of the exact answer.)

The table has a footnote that says "If your oven has a fan, reduce the recipe temperature by 68 F". I have a strong hunch that this footnote suffers from a translation error. How many degrees Fahrenheit should it have said to reduce the temperature by? (No knowledge of convection ovens required.)