HI there! My best advice to you is by using OPK's (ovulation predictor kits)!! :D You can buy them anywhere. You can get the strips which some think are confusing but I have used them or you can get the digital which either gives you a Positive or Negative, so there's no guessing. They detect the LH surge in your system and that tells you when you'll be ovulating. I usually start using them on CD9 which is cycle day 9, CD 1 would be first day of your period and I'll use them up until I get a positive. My period kind of bounces around too so I have been using the opk's for about 3 months now. :) Hope this helps and if you have anymore questions please feel free to ask. That's what we're here for. :) Good luck and best wishes!!
Most women ovulate about 14 days before their period is due, it can vary by a few days on any given month, but in general it's usually about 14 days.
You have cycles ranging from 25-29 days, so 25-14= 11, and 29-14 is 15. So I would assume you ovulate between the 11th and 15th day of your cycle. I suggest you have sex every other day from the last day of your period until the 17th day of your cycle, or at least the 9th day this way you cover the days you may possibly ovulate.