I was taught to do the calculation based on number of meals served. So for daycare it is 5 lunches and 10 snacks times 5 kids ( a snack equals one lunch) so that would be 10 meal units. Then do the same for your family including your own school age kids if you send food to school but not meals that say your husband or teens bought at work and aren't on your food bills. Include weekend meals for your family too. That will determine what percent of the grocery bill feeds the daycare and what percent feeds the family. Now mine is pretty much 50-50 but was more lopsided when my own 3 older kids still lived at home but then my bills were bigger too so the amount I wrote off was about the same. You only need to do the calculation once and use a typical week. Then at the end of the year just add up all the grocery bills and use the percentage. Remember to include your bills for pizza etc since those meals have been included in your totals based on the percentages.

I don't include major meals out to a restaurant because that totally messes with the averages but at the same time I don't sweat it if one of the girls comes and eats at the house as it balances out at the end of the year.