This app was mentioned in 3 comments, with an average of 3.67 upvotes
Legacy is a strange beast. Make sure you have Oracle access. Many venues have poor reception, so an app on your phone with offline access is ideal.
MTG Judge Core App for Android: https://play.google.com/store/apps/details?id=com.ndrue.gathereroffline&hl=en
MTG Guide for iOS http://www.mtgapps.com/mtg-guide/
This is the App that myself most Judges I know use.
https://play.google.com/store/apps/details?id=com.ndrue.gathereroffline&hl=en_US