The lump sum you enter is treated as supplemental wages. US supplemental withholding and employee FICA are taken off using the same rates as the bonus page. The cash left is divided by the monthly spending you enter to show how many months it covers. Unemployment benefits are not added.
A severance check uses the 22% supplemental rate when the amount is under $1,000,000, then Social Security and Medicare subject to the wage base. If $20,000 remains after those amounts and spending is $4,000 a month, the runway is 5 months from this check alone.
State unemployment is not added to the runway. A release agreement, a pension, and continued health insurance are not valued. This is not Canadian termination pay or employment-standards notice.