Sean I have those same books. Bought them when I was in my early twenties. The fundamentals are right on, but as advancement in technology and products evolve, so does the "parameters". My books also state a 15%-18% increase in enclosure volume is possible due to damping. Today we see figures as high as 30+%. It's just a learning curve. The books I have are: "Advanced speaker systems" (1995) and "Building speaker systems, second edition with automotive" (1988).
I would assume ratios change as product and r&d improve.