Of course the smaller the plane, the greater the amount of percentage error is a 100 ton mistake.
So what do you do to prevent this. We put the planned ZFW in the ZFW column of the FMC upon initial entry. Then when actual is read out, I point at the ZFW in the FMC and then enter it. A large difference should be questioned.
The OPT initial calculation is from the planned GTOW number on the flight plan(the ZFW number is not used). A significant difference here upon final calculation should also be questioned.
I also look at the V2 speed and see if it makes sense for general weight.
Any other suggestions are welcome. Obviously it can happen.