How do you account for a deifined benefit plan when figuring how much to save for retirement?

If it's really a defined BENEFIT plan, and not a defined CONTRIBUTION plan, you're ahead of the game. I'd suggest looking at what their current offerings are and approximating how long you likely will have worked for the company by the time you retire.

You should not assume you'll get a penny through Social Security. Even if the Ponzi scheme lasts much longer, there are a couple of major problems. First, I have no idea how old you are, but Social Security benefits will be reduced for those born 1950 or later:
http://www.cbo.gov/ftpdocs/60xx/doc6068/...
Second, Medicare premiums keep rising--up 120% from 1998 to 2008 and no reason to expect a change in that. You will also either need a "medigap" policy (ANOTHER insurance premium) or a restrictive HMO if you anticipate any real expenses.

You may have IRA money, savings, etc. but again, things depend on the future--inflation, tax rates, if you invested in stuff did it increase in value, hold steady, decline? When do you want to stop working? What will your health be like? Etc.

Right now typically people are told to assume they'll need $250K for medical spending in retirement. You can't afford to retire with a mortgage, student loans, or credit card debt either.

As you may have noted from some other questions on site, your Social Security even (should it exist) can be garnished for student loans and tax debts (as well as child support).

So while your plan is desirable, you can rest assured it won't be enough to retire comfortably on.

Regardless of benefits, you should aim to save at least 15% of each paycheck for retirement. That should provide you with more than enough funds to retire.