The rule has saved me extra money, I wish I could tell you but for the most part it's a simple way to save some real money at the grocery store. Here are some things that I don't buy any more and my reasoning
baking mixes, I know I have posted some before but you gotta love them. My reasoning is, if your buying a cookie mix, and you still have to dirty dishes, why not make a big batch of cookie mix and store it in a 20 cup rubber maid container. and yes it takes me about 10 minutes to make the whole mix, but I got 20 cups of mix or 10 batches of cookies at hand, and your saving a ton of money! Other baking mixes I have already prepared that I don't buy any more are: corn bread mix, bisquick, and pancake mix
Produce, some things I see working out while others not so much, bag lettuce is marked up at least 50% pre cut fruit and veggies, up wards to %100.
Fabric softener, like I mention in my last post
bread, it takes no time in the kitchen, and who doesn't love home made bread
Meat, buy meat when it's on sale and freeze it, 5min to repackage and divy out the portions, or cook the ground beef with what ever seasoning you need and you are all most done making a dinner to prepare, I like to buy 3lb-4lbs and cook it and then freeze it in four portions, so I am only dirtying up one pan one time instead of 4 times.
Cheese, my kiddo's love the cheese cubes, I hate the price, but for what seems like half the cost you can cut up your cheese cubes your self, takes 5 mins. I also like to grate my cheese, this takes me less than 5 min. to do.
I hope this helps you to save some extra money at the grocery store, like me this was an ah ha moment that I thought was a simple way to save some money.