what's the Vref 40 rule?
Vref 40 is calculated and displayed by the OPT on the iPad based on the weight entered. ( in this case 66400kg).
At the same time, Vref 40 is calculated by the FMC based on the weight of ZFW entered + the actual fuel being sensed.
A check of the OPT v's the FMC Vref 40 is required after the independently calculated figures to ensure that they are within one knot of each other.
Basically it means that the info you have generated on the iPad is using the same gross weight that the FMC is using.
The weakness in this check is that if the wrong ZFW is entered in the FMC they can agree. That is why the FMC ZFW and gross weight is verified by both pilots from the final load sheet.
If rushing is avoided, the system works well and it is common to catch errors. I would guess that every few months I find that the system catches an error, it is usually a very small error. And always in my experience only one pilot has made that error.