for my business, I multiply the gross x .0765, which is SS and medicaid. I withhold Federal tax Per W4. FWIW, I keep an I-9 on file with a photocopy of a drivers license and SS card (or other suitable ID). Prior to the 15th of each month, I make a federal deposit with wachovia for 2x the SS/Medicaid + the Fed tax withheld. Every quarter my accountant comes in and corrects any errors and files the UCT6 form.