great, thanks!
here is a gif of the final result:

i want to have control over the first and the last amount via two text layers. i use a slider to let the amount fall. so when i write a number in the first text layer, it should effect the first keyframe value of the slider and the second should effect the second keyframe value.
the goal is a bulk production, where the composition gets feeded by a google spread sheet. the script i use for this is called templater from dataclay.
mark