Taken from the pages of Taking Charge of Your Fertility
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How to determine your temperature coverline:

1:  Identify the first day your temperature rises at least 2/10 of degree higher than it had been the previous 6 days.

2:  Highlight the last 6 temps before the rise.

3:  Locate the highest of those six highlighted temps.

4:  Draw the coverline 1/10 of degree above the higest of that cluster of six days preceding the rise.

5:  High temps during your period are irrelevant.

6:  Once you see a third high temperature above the coverline (the 3/10 higher than 6 preceding days) you know you have ovulated.

7:  For daylight savings time, take your temperature 20 minutes earlier
Saturday 4/3, 20 minutes later Sunday 4/4 and then take temps normal time on Monday.

NOTE:  Most NFP books will tell you that when you see above coverline temps for 18 days you are pregnant.