Improved Evasion (available on at least Rogues and Swashbucklers) provides
You elude danger to a degree that few can match. Your proficiency rank for Reflex saves increases to legendary. When you roll a critical failure on a Reflex save, you get a failure instead. When you roll a failure on a Reflex save against a damaging effect, you take half damage.
How much damage does such a creature take on a critical failure? Full or half?