Hi! I've usually got between a 40-45 day cycle, and I ovulate about 12 days before the end of the cycle (So O around day 28-33). Most women ovulate from 12-15 days before AF arrives. The luteal phase (LP) is the phase from ovulation until your period arrives. That is usually more consistant than the time before ovulation - that is more likely to fluctuate with stress, etc. An OPK will help, but if you keep an eye on your CM and your temps, you'll probably catch the right day! BTW, FF gives the green light to me any time I enter eggwhite or watery CM. My guess is that you are actually not fertile yet, if you do have a 40 day cycle. More likely to be around day 25 or so. But as you said - it's fun to try anyways! Can't hurt!
HTH!