i use this
public static function pixelPerfectCameraValue(camera:Camera3D,h:Number ):Number {
var h:Number = h;
var fovy:Number = (camera.lens as PerspectiveLens).fieldOfView*Math.PI/180;
return -(h/2) / Math.tan(fovy/2);
}
move the object on is back vector with this value
ie:
var positionCamera:Vector3D = Matrix3DUtils.getForward(camera.transform);
var newPos:Vector3D = new Vector3D(camera.position.x + positionCamera.x * -(Utils3d.pixelPerfectCameraValue(camera, 450)), camera.position.y + positionCamera.y * -(Utils3d.pixelPerfectCameraValue(camera, 450) ), camera.position.z + positionCamera.z * -(Utils3d.pixelPerfectCameraValue(camera, 450)))
dont know if there is another method, maybe more correct.