Wait, was it Calm or Mild, lawl? o.0
Just use the IV to PID site and type in IVs+nature and look for Method 1 for legality. No possibilities and Method 2/3/4 indicate illegitimacy/hacked or illegal IVs for the in-game legendary.
Calm flawless is legal, mild flawless is illegal. I guess I was reading another nature, lol.