While you are on the site, in your browser find and click "Add to Home Screen" A CGN beaver app icon will then be created to your phone that is directly link to the site.
Note: This feature may not be available in some browsers.
I usually figure mine beforehand with JBM Small Arms Ballistic Calculator (it is free); http://www.eskimo.com/~jbm/calculations/traj/traj.html
I print the tables and take them with me. I guess you're looking for something portable though...
Do you use a mildot reticle? The Mildot Master; http://www.mildot.com/
is a pretty cool ballistics calculator. Cheers