When am I legally allowed to panic?
When the fin is triangle shape and between 500mm - 1m protruding from the water.

Even then no need to panic because they normally won't let you see them if they are going to attack. If they are hungry they will hit you like a mack truck, it'll be out where the water drops away to the dark depths of the indian ocean. That way they can generate speed from below, it will be over in minutes.

That's my uneducated guess anyway......