The measurement method measures the maximum of the highest and lowest leads of the device when the device is on a flat surface.
 
Please refer to Altera's URL link below for this information.
  https://www.altera.com/support/support-resources/knowledge-base/solutions/rd08161999_686.html
 
For the flatness of each package, refer to "Altera Device Package Information
Data Sheet" and search for the keyword "Lead Coplanarity".
  https://www.altera.com/en_US/pdfs/literature/ds/pkgds.pdf