Mature root length for Ichi-Ki-Kei-Jiro

Anybody know root length for mature Ichi-Ki-Kei-Jiro? I know it is at least 12’ one direction, because I dug out one when I dug garden bed near my persimmon tree. But I consider my tree still young. I am looking for the best placement for new tree in my new house and the best spot is in some proximity of the leach field. Don’t want problems to the septic