jmedia -
I started with this image :
Based on this, I (arbitratily?) chose a rearward center of roof camber :
In the above, the red vertical line is what I *think* is the center of roof camber. Maybe aerohead can chime in. Then I applied a personal rule that the spoiler should not extend past the rear bumper. That is how I defined the red box in the back. Where the red box meets the slope is where the spoiler should extend. By sheer accident, that line is the bottom of the van window behind your Escort. If I am correct in determining the "center of roof camber", then that leads me to this mock-up :
I don't really like it, but there it is.
Again, if it's correct, then what I am suggesting *might* be the inspiration for the "Sierra RS Cosworth whale tail" :
Steve's 1989 XR4Ti
A less(?) agressive solution may be applicable from the Merkur XR4Ti :
1987 Merkur XR4Ti - 1980S - Jalopnik
CarloSW2