It might take some experimenting, but you should be able to figure out a pattern. For a while I wasn't getting good sleep while TTC but I found out that no matter what if I took my temp at the same time everyday it would be fairly accurate. I had some days where I had to be up at 5 am for work and those days would always be off, no matter how long I'd slept prior to temping. For some people the time is more important than the duration of sleep before taking your temp. Play around with it. It may take a couple of cycles to figure it out. Since you've temped and charted before you will hopefully be able to pinpoint things easily.
I love reading charts!! If you do use FF post a link to your chart so I can stalk it