You should use the accounts only for expenses you know you will have. Don't guess or put in a little extra for emergencies. And, remember, you are saving money on taxes. Even if you forfeit a little bit, you might still come out ahead. I don't like to plan.
Planning is difficult for many people, but it's a necessity.
Planning health and day care expenses is relatively easy. Take a look at your health expenses for the last couple of years. That will give you some idea of the amount you typically spend and the types of expenses you have. Think about whether you are likely to have any of those types of expenses. A few quick calls to doctors or other providers will get you the cost.