  We provide a simple and predictive random-matrix framework that naturally
generalizes Page's law for ergodic many-body systems by incorporating a finite
entanglement localization length. By comparing a highly structured
one-dimensional model to a completely unstructured model and a physical system,
we uncover a remarkable degree of universality, suggesting that the effective
localization length is a universal combination of model parameters up until it
drops down to the microscopic scale.
