My doctor says, the time between your ovulation and start of period is always fixed at 14 days, but the time between your end of period and ovulation could vary. (Just as 9Rmine says above). So count backwards from your next expected period. I am trying to test this out myself as we have been trying to concieve also. I tried the kit in December but didn't get any surge detection at all... until I ran out of all the tests. Turns out I ovulated much later because of stress and my period was delayed for 12 days. Am planning to try the kit again next month, if I dont get pregnant this month.
at 11:18 AM on Feb. 12, 2009