Since me and my girlfriend started living together, we put money into a shared account for grocery shopping.
Our first estimate was, that we required 200 euros. That turned out to be woefully inadequate, so we kept increasing the amount until we hit 400. When I googled for the average food spending in The Netherlands, it turned out to be 350, so we lowered the budget. Totally confident that by decreasing the budget, we'd also spend less.
Actually, our spending increased. Since January, we spent the complete budget of 350, and had to add another 100 in the last week of the month.
I'm sick of this, there's no reason we'd have to spend so much. Thus the challenge is: twice every month, write out a menu for two weeks, and make sure it does not add up to more than 100 euros. One groceries chain will deliver that to the front door, for an added 4 euros. That's total of 208 per month. We won't hit the shop, unless it's for fresh veggies, yoghurt and milk.
That should limit our groceries budget to a maximum of 250 euros.