Potassum-Argon Dating and Proportions of Daughter Products in Potassium 40 Decay Channel
Potassium 40 has a half life of 1,300,000,000 years,  which means that within that time half of the 11 percent which you refer to will have decayed to form Argon 40. Potassium 40 has two modes of decay, which is unusual for a radioactive isotope. Only 11 percent of it becomes argon 40, the remaining 89 percent decays by a different mode and becomes Calcium 40. but there is no need for you to worry about Calcium 40, it is the 11 percent of K40 which becomes Argon 40 which is useful in dating ancient rocks. Fortunately geologists don't have to wait until it has completed one full half life for it to be useful. By assessing the amount of Argon which has accumulated within the material since it was ejected by volcanoes in pristine condition and had no Argon at all in it, geologists can get a rough idea of the age of the rocks. The potassium/argon method is very tricky to carry out and error can easily creep in.