Surface of the world’s oceans in volume assuming a 20 foot deep layer and adjusted for the loss of volume when ice melts into water.
This is of course inaccurate since no one factored in the INCREASE in surface area due to the rise in water level and the subsequent area flooded by the inundation.
In other words, the calculation only allows for the water to move upwards (20 feet)at the current coastline but does not account for the area inland from the old coastline that will be under water, for instance, the whole of Holland could be under water...