This is Spocalc2014.xlsx, written by Damon Rinard.  I updated it to the latest Excel version and replaced macros with data validation so that it would run as a web app.  Other than that, this is the same awesome spreadsheet I’ve been using for over a decade.  It’s fully functional right here on the web or you can download it to your machine by clicking the Excel icon at the bottom of the spreadsheet.   Visit http://www.sheldonbrown.com/rinard/spocalc.htm for detailed information on how to use this tool.