@KarenOLane if it is a 401k plan, then it IS subject to FICA and Medicare, so the rule you used DOES do it correctly by reducing fed and state w/h but not FICA and Medicare wages. We've done test payrolls to confirm it is correct - if it is a 401k deduction, box 12 D code we use -805002.
From the IRS web site - Topic Number 424 -
401(k) Plans
A 401(k) plan is a qualified deferred compensation plan. If you're eligible under the plan, you generally can elect to have your employer contribute a portion of your compensation to the plan on a pretax basis. Generally, your deferred compensation (commonly referred to as elective contributions) isn't subject to income tax withholding at the time of deferral, and you don't report it as wages on Form 1040.pdf, U.S. Individual Income Tax Return, since it isn't included in box 1 wages on your Form W-2.pdf, Wage and Tax Statement. However, it's included as wages subject to withholding for social security and Medicare taxes. In addition, your employer must report the elective contributions as wages subject to federal unemployment taxes. Some plans also allow you to elect to make your elective contributions on an after-tax basis as designated Roth contributions
These comments are scaring me into doing more research since we've already upgraded several clients!