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 highest 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.