okay solved the problem now with a Slider from 0 to 100, some expressions and a linear-function
p1 = thisLayer.height-thisLayer.height;
p2 = thisLayer.height;
p0 = thisLayer.width/2;
slider = effect("Slider Control")("Slider");
linear(slider,100,0,[p0,p1],[p0,p2])
i combined that expression with a linear wipe and got a nice drop in animation without an extra Matte Layer.
but my second question is still standing, can i define the value of every keyframe i set in my expression?