In general, a helpful trick for grocery shopping is figure out approximately what you spend in a year on groceries and divide it by 12. Then put that money in the bucket each month (meaning some months you will spend less than you are putting into the bucket). Then, at the beginning of the month when making your budget for the month, check how the calendar falls (for example, if you normally spend $100 on groceries on Thursday, then check how many Thursday's are in the month and budget accordingly).
This method also helps if you have extra, large grocery costs some months (such as stocking up on a meat sale or renewing a Costco membership). Then, you add in a couple hundred $ extra at the beginning of the month into your budget, and since you are putting away monthly, IYH there should be money in the account to cover it.