How do I know if my baby has a fever?

My 10 week old baby has been coughing all day and her temp taken orally was 99.8 and I am not sure if that is considered a fever. I know whether you take the temperature orally, rectally, or under the armpit you need to add or subtract one degree. If taken orally do you add, subtract, or do you just read what's on the thermometer?