I had similar problems with Anna. She would always wake up at different times, I never knew when we would start our day. I tried and decided not to wake her up, she doesn't like it, and neither I. If I woke her up at a set time, she was fussy and unhappy, I obviously broke her sleep in a wrong moment, and I can understand that, I still remember how I disliked being awaken by my mom.
So I just went with a 3 hours since she woke up. This gives you kind of a schedule, you can predict when her naps will be. But, because each day they are at different times, her body clock doesn't learn to go to sleep at certain times, this is more of a problem when she will get older. I started to be really annoyed by this when Anna was on one nap a day and still was having it all over the place. One day, she went for a nap at noon, the other at 2 pm. So it is basically up to you to decide what is the best for you and your dd.