According to Clinical Gynecologic Endocrinology and Infertility (page 1285), the egg survives in the female reproductive tract only for about 12 to 24 hours after ovulation.
The calculator assumes ovulation occurs 14 days before the date of an expected period as explained in the section on 'Calculation by Menstrual Cycle Length' above.
If you were charting increased pain at mid-cycle, the day of the worse pain on one side of your pelvis is the likely day your egg ovulated.
This is probably the day you conceived if you had intercourse around that time.
The day of your definitive positive reading is the day of the biggest increase in your LH.
This "LH surge" triggers ovulation and predicts it is likely to occur 24 to 36 hours later.