Great Idea!
I also try to stick to approximately $400 a month. I say "approximately" because my budget works slightly different. I am NOT good with budgets at all. So instead of going month to month, I go paycheck to paycheck. We take in bi-monthly, so every 2 weeks I hit the grocery store.
My budget every two weeks for our family of 6 is $200.00 (which most months is $400/month ~ except for the 2 months a year we get a 3rd check ~ confused yet?? I am too actually

) Anyway.....
This includes ~
all food I prepare in the house, lunch food for Dh & kids, and snacks
all cleaning products and beauty needs
Diapers and wipes for Josie
most often food for the cats
Last shopping trip I went overboard and blew the $200 slightly. HyVee was having an AWESOME sale so I bought more than I planned to, however, I know that we have tons of meat now ~ so my spending will be less next shopping trip ~ so it all evens out.
$200.00 (start budget)
- 94.35 (groceries/meats)
________
105.65
-103.30(WalMart balance of groceries)
________
2.35
-11.46 (Kwik Trip ~ Milk, bananas)
_______
-9.11
So I am currently $9.11 over budget this time, and I still need to go get milk this morning. But then I am good until next Friday when I shop again.