IMHO, healthy koi need extra salt like they do a bicycle.
They should get all they need from the water changes and food.
Use salt if and when you need to treat a koi for a parasite or infection. You can find info on that in books such as The Koi Doctor, Advanced Koi Health Care...etc.
Instead of focuing on salinity levels, measure the pond ORP and try to maintain it over 300mV. With high water quality and good ORP...the koi will not need an extra salt or other routine chemicals added.