For federal tax withholding, use Publication 15: http://www.irs.gov/pub/irs-pdf/p15.pdf
If you read pages 36 - 39, you'll be able to calculate your federal tax withholding exactly.
Social security tax is 6.2% of gross compensation up to a limit of $102,000 of compensation
Medicare tax is is 1.45% of wages with no limit.
I've got an Excel spreadsheet set up to do the same thing. I can enter each gross paycheck amount, the spreadsheet does the calculation, and I can forcast what my various withholdings are. That way, I can be sure whether my taxes paid for the year will cover my taxes due when I file.