Can I file as Head-of-Household if I and my dependent child moved out of my parent's house mid-year into my own place

Consider the following scenario - an unmarried adult (Sue), with her own dependent child, lived for a few months in the home of her parent;  then she and her dependent child moved into a separate household for he remainder of the year which she  maintains independently,  Can Sue then file as Head-of-Household.  The IRS rules say that to file as Head of Household, you have to:

• Pay for more than half of the expenses for a qualifying household.
• Be considered unmarried on the last day of the tax year.
• Have a qualifying child or dependent.

How does one figure  "half of the expenses for a qualifying household" when there's more than one household?  Sue's parent did not claim her as a dependent during the period that she co-habited and Sue bought her own groceries, contributed to utilities, insurance, taxes and helped maintain her parent's household.  The she moved out with her dependent child and independently assumed the full cost of  her second household for the rest of the year.  The IRS rule says "This portion must be for the entire tax year." but how should this be apportioned or otherwise be pro-rated when the expenses vary over two different households?