So THAT'S why you're struggling with it now. I was trying to figure that out.
From my experience in the past (multimedia fusion), working from the image point is not the way to go. you leave that on the base of your object.
I start out with a center point sprite that represents the point of origin, when the object rezzes at the horizon, the X and Y instances are set to their destination X and Y (where they will be when they reach the front of the screen. the Z variable will be the percentage of the path from the point of origin to the destination. when an object is at 0, it will be a the horizon, and 100 will be at the front of the screen, scale at max. objects on the ground have a Y value as the bottom of the screen...
the hard part is getting the formula right to make it look like 3D. when I'm not busy, I'll send you what I know