I was told that if you use your whole house for daycare and have to use the formula to find the percentage that you can write off household bills for than you would need to use a dollar amount/per day/per child and keep a record of how many snacks and meals you have provided over the year.

If you use half your home for dc only than you can claim 50% of household bills and groceries.

This is what accountants and other hdcp's have told me so I would suggest asking your accountant which way is best for you.