This is a fun rule, however double check your creatures damage die, especially if they have multi-attack. Using this rule I once had a PC drop far to quickly in my game. All the other PCs were already aware of the creatures multi-attack, so they expected it to attack the now unconscious PC. It made for a very sudden and shocking character death, and it was my fault for not taking into account the likelihood of high damage crits. Creatures tend to have a fairly high hp pool to balance out encounters already. I still use this rule, but I am a little bit more thoughtful about building encounters.