Fixed percent based checking; updated usage

Hello,

We found your script, and really liked it, however we required percentage based checking to work.  I went ahead and made some minor fixes to get the percentage based checking to work, and updated the usage documentation to be a little more clear about the options, and also included the information for percentage based thresholds. Hopefully you'll approve of these changes!

Thank you,

Michael Rollins
System Administrator
Indeed.com