((Perhaps using a set number but not quite 20, like in the 15-18 range?  Within that range, a pred of equal skill to his swallower still only escapes less than a quarter of the time, resulting in frantic effort to escape but still within the realm of feasibility.  Comparably, this would also mean that less skilled predators remain swallowed longer, edging towards 'needs a natural 20' quickly.))