The coverline is a horizontal line drawn after ovulation to help differentiate temperatures before ovulation (low) and temperatures after ovulation (high).
Your coverline is drawn after you have clearly established ovulation.
When your waking body temperature rises more than 0.2 or two-tenths of a degree higher than the previous six days, and it stays that way for at least two days, your coverline can be established. This rise normally occurs during your thermal shift the day after ovulation.
For example, a jump from 97.1 to 97.3 is a rise that is two-tenths of a degree higher.
FYI, you won't be tested on this, and best of all, your BabyMed chart will automatically calculate your coverline once you've entered the proper data. Everyone say it now: "Whew!"