The post on Display Viewing Calculations from has been one of the most popular on this site, and we decided to make the calculations a little easier to perform for everyday use.
Since our team also develops mobile apps, the best way to make this accessible to everyone was to create an app that would let you do these calculations in the field in real time.
Check out AVDistanceCalc in the Apple App store.
Also available, AVDistanceCalc in the Android Market.