It is better to underfeed than to overfeed. Koi are foragers, and if your pond is the ecosystem variety, there will naturally be lots of food for your fish to eat among the plants and rocks. To keep the system in balance, a good rule of thumb is to stay at 3" of fish for every 100 gallons of water in the pond. Adhering to this rule will ensure that you don't have more fish waste being created than the filtration can process.
Now, if you would like to learn more and go in depth about feeding koi, I suggest reading the following page by our go-to fish expert, Dr. Erik Johnson. He has written a very thorough article all about koi food, koi eating habits and recommended feeding practices.
Everything you need to know about feeding koi, by Dr. Erik Johnson.