The lump sum payment should be added to all other taxable income to determine your tax bracket rate for the year.
Taxable income can also be looked up on the tax tables listed in the instructions for form 1040 (available for download at www.irs.gov under forms and instructions). This will give you the tax for the year. Subtract any withheld tax from paychecks, lump sum distribution (usually 20%) , estimated payments, etc to see how much will be your refund/balance due. You can adjust your withholding by increasing it if you will have a balance due or reducing it if you will have a refund.
Remember that a tax refund is paying yourself back YOUR OWN MONEY which the government is using interest free while they hold it for you.