That is unusually low. I've noticed that 50/50 jungles bred to 50/50 jungles can have rates that low (not always, but I've seen quite a few), but usually a corn to a king will be about the same as corn x corn or king x king.
Did you breed them once or twice? A single breeding rather than a double could be the cause of your low percentage.
Also, was it the first year for either snake? That too could cause such issues. I'd try the same thing next year and see what happens.
Meanwhile, congrats and please share pics when you can!