The answer may depend on how you want to use the data. Do you want to include the metadata for the file, typically the inodes? There's a metadata option in fsmap as well. That will normally be small relative to the file size, but if you have small files that are highly fragmented, perhaps it might become material. Also, how often are these files changing between when you run the command and when you use the data for whatever purpose? Thinking through other possible issues, if you have sparse files, I'm not sure whether holes are counted, although I suspect not.
Can you describe your goals a bit?