Well, 40% of height would be right around 30" for me. I'm not sure that I had a 30" waist back when I was running marathons in my twenties; 32", maybe. At this point, I don't think anyone this side of Procrustes could get me into trousers with a 30" waist; yet I think I'm reasonably fit for my age.

The USMC calculator for body fat uses neck and waist circumference both, and by that I was (last time I measured) in decent shape for body fat percentage.