If you stick to percentages, yeah, the size has to be defined before you can know how much of it is being used. But if the information was in pure units of area (square centimeters, square mils, nano-acres, or whatever your favorite units are) it could be useful in the early stages of a design process when you’re still asking questions such as, “How large must the enclosure be?” or “Can we find space to squeeze this additional circuitry into the existing board outline?”.
After you get to the point of actually placing components and routing traces, a feature like this may have some kind of gee-whiz value for the marketing folks but I don’t think it has much practical value. Even as a tool for planning and budgeting, the algorithm would have to account for factors like aspect ratio (e.g., in most cases a 1 cm by 9 cm board is less usable than 3 cm by 3 cm), board edge setbacks (0.25 mm all around, or whatever the design rules require), and a general “packing efficiency” factor.
Dale