MatchRate gets the rate of change between two keyframes. Using that rate, MatchRate (a free After Effects script) can create new keyframes at the same rate. In addition, you can apply the rate to other layers in your project.
0:50 Extending Animations
2:10 Applying Rates to Different Properties
5:30 Animating Ratios
6:50 Cinematic Titles
7:53 2D Parallax
Also available for download on aescripts.com/matchrate