Yea don't go buy the 14day rule I ovulated really late last cycle and if I didn't use Opks I would have missed it, so your best bet may be either temping or using Opks to make sure or pay attention to your CM
You can't go by the 14 days rule. My periods are 25-26 days apart and I ovulate a week after my period. Use this app and count how many day are between your period. Then make sure you change the setting from 28 days. It should have a more accurate ovulation time then. If all else fails, buy a ovulation kit.