I have a child with unpredictable days as well. I insisted that they give me at least 14 days per month, and they were happy with that. They pay a flat weekly fee, regardless of his attendance. This means that he doesn't have to pay stats, but they also have to pay in full if I take vaca days. I determined their flat weekly fee by multiplying the number of days per month by my daily fee, then multiplying this by 12 (months per year), then dividing by 52 (weeks per year). It works out very well for us.