I charge one rate for full time and half that rate for part time. I won't accept just any part time schedule (aka..schedules right smack in the middle of the day) but currently I have one dcb that comes from 12-530 and I could take one morning part timer if someone needed. Which they haven't! I really don't like part time schedules because like you were saying, now I have to find a part timer for mornings...but I wanted the sibling set. One was full time, and his brother was just afternoons, after pre-k. I used to charge hourly trying to accommodate parents that only needed a few hours a day, but decided that it wasn't profitable because the child was taking a full time slot. So....now I have full time-anything over 25 hrs, and I decide depending on the children in care, whether or not it is worth it to take a part timer.
