Ovulation help

So I started tested 2 days ago like glow said and I keep reading everyone say to not use your fmu, but my question is I don't have a regular sleep schedule I work 12 hour over nights so I'm not sure if my fmu would be when I got off work or when I'm first waking up to go to work. It's very frustrating because me and the fiance have been trying since August with no luck. He is becoming very discouraged about it all. Please help