I am certainly not an authority on snake internals. I think the best answer will come from analyzing what your type of kingsnake would feed on in the wild as a hatchling (i.e. lizards) and then compare the calcium content of lizards vs pinkies.
On the other hand, many thousands of hatchling kings, milks, corns, etc have been raised to healthy adulthood with no supplements and a diet of soley pinks as hatchlings. So we know pinks alone are a pretty good food source just by practice.
I have heard some theorize that female snakes of species that would normally eat lots of lizard prey in the wild (like mtn kings) do benefit from calcium supplements if being bred. Whether this is true or not I don't know, but one breeder claims he has had better results doing so.
This is a great question for some of the more biology/physiology minded folks on these forums.
I think it is great that your snake is eating now.