zooming as in constant z movement

Hi, I'm trying to zoom in on a plane as if it was moving towards the camera
at a constant speed. (I can't use z position in this case). If I use linear
scaling for this, the object looks like it moves very fast in the beginning,
and slowing down as it approaches the camera.

I think this problem is very common zooming problem, with a nice and easy
math solution, but my math skills are lacking here:-/ Can someone enlighten
me on this one? Thanks

-Thijs