Wonky cycles OPK question..

Jessizu • I wear lots of hats.

So ladies my cycles are wonky... they are sometimes a perfect 28 days to 40 days.. last cycle I ovulated on day 28.. the cycle before it was cycle day 15... i was told I'm ovulating from my OB from ultrasoind.. just trying to see what day I should start using opks... i have plenty of opks.. what would you do if you were me?