Right Angle App

http://itunes.apple.com/us/app/right-angle/
You can download this app to your iPhone for free.

Settings
Give results with fraction -ON
Give results in FT & IN -OFF
Precision -16ths
Angle Formatting -Degrees

Enter any 2 values and press the = sign

Slope is the same as run (R)
Rise is the same and height (H)
Base is the same as length (L)
Degree is the same as angle (AA)

Your values should agree within about an inch. Your Angle should be correct within 1 degree.

If they do not agree, try using the sum of the risers and/or the sum of the treads (if they are different) instead of H and L.