How do I estimate W-2 numbers using my last paystub?

While waiting for my W-2 to arrive, I would like to enter estimates into my tax program so that I can gauge my tax liability. (Of course, I won't file until I've received the actual W-2 and adjusted my estimates). I found some basic info about estimating the W-2 numbers... show more While waiting for my W-2 to arrive, I would like to enter estimates into my tax program so that I can gauge my tax liability. (Of course, I won't file until I've received the actual W-2 and adjusted my estimates).

I found some basic info about estimating the W-2 numbers here:
http://vpf-web.harvard.edu/ofs/payroll/emp_w2.shtml

It appears I just need to subtract pretax items from my gross income to get box 1 and 16 (federal and state taxable wages). But how do I calculate social security / medicare income? The web site above implies that it is somewhat different, but doesn't detail what the differences are.
Update: The reason I'm trying to estimate all 4 numbers (federal income, state income, social sec income, medicare income) is because the tax program won't let me continue to the next screen without that info.