Say i have a layer at 50% scale. I move it back 1000. now what is its true scale value? Of course i will usually just eyeball it. If its too pixelated i undo or change out the shot. Really im just curios.
scale = Focal Length / (Focal Length + z depth into screen)
In Ae, the 2D scale equivalent of a reference 3D layer, roughly expressed for a default camera:
l = thisComp.layer("3D Layer");
focalLength = thisComp.activeCamera.zoom;
s = focalLength / (focalLength + l.position);
[s,s] * l.scale;
This covers the simplest case only. It doesn't deal with non-centered anchor points or parenting, nor does it get into the vector math for calculating the distance of a layer down the camera's Z-axis. We can re-visit these topics, too, if you need to.