Allow negative balance

Summary: I am wondering if there is way under the accrual tab if we can allow negative balance but reference a fast formula? I am wanting to allow negative balances for employees based on their ceiling. For example if the ceiling is 80 hours they can go -20 , another example if the employee has a ceiling of 40 hours then they can go -10 with the balance. We currently have allow negative balanced checked and it can go up to -999 but we are wanting to have a rule that references a fast formula where we can code it