What you describe is how the Zone tool works as well. But by using Fills instead you loose some of the power of Zone tool particularly in this case where the original query was how to calculate the Nett and Gross areas.
The Zone tool can do this quickly for you as I have shown above. Whereas to do the same thing with the Fill tool requires editing all your Fills and creating a second set of Fills to calculate the Gross.
The Zone tool also has the benefit that it can feed information into Opening and Finishes Schedules which save time and effort. Whereas the Fill tool can't feed Room Names, Wall and Floor Finishes into schedules
Don't be scared of the Zone tool, embrace it and make it your friend