However.the scavenge shut-off valve [float operated] will not be open until #1 main tank fuel qty is below 1900 KGS
That does not fit with my experience on 732/3/4/5, eng. Is it NG type specific? It is understood that it is designed to EMPTY the center tank when the ctre tank pumps dry. At this stage there will probably be around 4000+ in each main.