Can we use Mom's money for part of the food bill and utilities?

We have guardianship and our Mom is living with us. We don't mind having mom with us, however, we can't seem to get an answer as to how to handle food and utilities bills from a reporting situation. We are tracking all of mom's expenses for Dr. visits, Rx, and personnel services with receipts. Is there a reference of how to handle dividing up food costs? or increased utility coverage? May we take receipts and divide by number of people in the house?