Getting Ahead in YNAB

One of our financial goals this year is getting a month ahead. We are already a month ahead on fixed bills, but I’m struggling with how to get ahead on the variable ones (groceries, gas, sinking funds, etc). How do you know that what you’ve stored for the next month is enough to cover these variable categories? I’m still in my first year, and doing a better job of curtailing spending, but I still end up WAMing some of these variable categories, which can be higher or lower depending on the month. I have been budgeting next month bills directly into the next month on YNAB. Thanks! Merry Christmas!