I'm not sure I understand what you want to do. If FAR is calculated for the building as the total floor area divided by the lot area, then there is one value for FAR for the whole building. How do you want to calculate FAR per floor? Do you want to consider only the floors up until that floor, meaning that the floors above that floor do not count? For example, if the building has 5 floors total, would the FAR value for the second floor only consider the areas of the first and second floors, disregarding the third, fourth, and fifth floors? In that case, you can change your calculation of FAR to use split.index.
FAR = geometry.area(bottom)*(split.index+1)/ParcelArea
This gives you the floor area of one floor multiplied by the floor number all divided by the parcel area. Note that split.index starts at 0 for the ground floor, so add one to get the floor number.