I’m with Eldred , almost. I would say start with $750, or maybe $725, and then try to beat it.
Also if you haven’t already I would separate groceries from takeout and restaurants. It’s easier for me to see what the tradeoffs are if I keep those separate.
I have a teenager too and even though she’s a small girl, she can really eat!
It helps me a bit to understand why the big bills are big. Am I stocking up on staples because of a big sale or have I gone overboard with junk food and prepared food? I also bought some food specifically for our teen guests to keep in the basement – large bulk boxes of soda cans and the large size of ice cream (my daughter and I are happy eating single-serve ice creams but a gang of teenagers will plow right through those!)