The best scenario, is to end up owing $0 or as close as possible by the end of the tax year. Of course, this is quite tricky if you are married (and/or have kids), have more than one job, or other income, dividends, etc. that will make it very difficult to get near $0 liability.
Your employer’s payroll department has the 2004 “Circular E, Employer’s Tax Guide” aka IRS Publication 15…ask to see it and look (in the tables) what your deductions will be if you claimed Single, 1 dependent v. Single, 0 dependents for your comparisons. Or, you can look it up here. This is a pdf file.
Also, rather than claiming a certain number on your W-4, you can request an exact amount to be taken out of your check each payday, therefore taking out an expected amount out of each check, each pay period.
Example: If your total tax liability (not just what you owed [or even refunded], but your overall taxes owed for the year) last year was $2400, and you are paid twice monthly, and expect little or no increase in salary, then on your W-4, claim the max # of dependents and on line 6, “Additional amount, if any, you want withheld from each paycheck”, enter $100 (which is $2400/24 pay periods). You should see $100 dollars deducted from each check. If you are starting now to have more fed tax taken out (let’s say there’s only been $100 taken out so far), then since there are 17 paydays left in the tax year, then divide ($2400 - $100)/17 paydays left ~$135 each for the remaining pay periods.
This is a simple calculation for a single person with no other dependents, more calculations are needed for children, spouses, 2nd jobs/incomes/dividends, etc.