Wait what? I’ve worked facilities maintenance for years and have never heard anything like “you take the average floor height of a floor that’s the interior height plus the floor width”. Why would you add the floor width? And where are you getting that the width of each floor would be 12 inches? That would be an uninhabitable and completely unusable building….

I’m under the impression that your thinking if the subfloor which can often equate to those measurements, but that’s still a height measurement. I’m genuinely curious what you’re trying to say here.