Start with the disruption you are solving
If the fund is for a broken car, a job search, or a medical bill, name the risk before choosing a target. The more specific the job, the easier it is to tell whether the balance is useful.
Use ranges instead of a magic number
Consider essential monthly expenses, how predictable your income is, the number of people relying on it, and how quickly you could reduce costs. A range can be more honest than a universal rule.
Review after a life change
Move, marriage, a new child, a job change, a health event, or a debt payoff can change the right reserve. Revisit the target when the household changes, not only when the account feels low.