As far as I am aware, the zooming function is not able to zoom to align blocks on the top left. The ZoomFactor parameter only has the below options, and FitSystem/FitSelection will both try to center the system/selection.
I am not aware of any other commands to move the viewbox in a more precise way.
To zoom only the top-level system, the command is:
set_param(bdroot(gcs), 'ZoomFactor', 'FitSystem')