so animate the offset factor (with fixed position toggled) 0 at 0, 1 at 100frames. set curve to linear. set key at 40frames. copy value (ctrl c). paste value (ctrl v) at 60frames. set key. move last keyframe with y constrained until you get similar slope to the first 40 frames. { .2 change in 20 frames. rise over run would be, .2/20 }