The top section is where you enter how much you would like to actually "get" after all PayPal fees are taken from an amount. The second section is a simple fee calculator where you enter how much you would receive, it would tell you how much the fees are and what you would get after those fees are removed.
important disclaimer stuff:
This calculator is provided "AS-IS" and I don't guarantee it's effectiveness nor it's accuracy. This site has no affiliation with PayPal Inc. or any of PayPal's affiliations. If you discover an an error, by all means let me know so I can adjust it for everyone.
Updated February 15 2015: Since 12/02/2015 - the new default percentage on fees has been raised from 2.4% to 2.6%.