Using 529 for room and board when FT student living at home

My son is attending community college full-time while living at home. From prior posts, it looks like we can claim room and board up to the college's allowance for cost of attendance. My question is, we have 4 of us living here. When we calculate the room and board (like utilities, food), do we need to take 1/4th of those actual expenses? Or can the entire amount be considered a qualifying education expense?

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