It seems to work. And it’s a lot easier, than I expected it to be, but I never came up with the idea of using the zoom.
Thank you a lot Darby!
Here is my, probably not very well written, expression.
C = thisComp.activeCamera.toWorld([0,0,0]);
P = thisComp.layer("3D layer").toWorld(anchorPoint);
dis = length(C,P);
CZ = thisComp.layer("_Camera04").cameraOption.zoom;
S = thisComp.layer("3D layer").transform.scale[0];
temp = S*CZ/dis;
[temp,temp]
and of course this one for the position
l = thisComp.layer("3D layer");
l.toComp(l.anchorPoint);
http://www.schwarzgrau.com