For those that do use chairs, I personally prefer to keep the chair as low as possible to avoid having to raise the left foot higher which can lead to a lot of discomfort. Also allows the right leg to have a more active role in supporting the guitar. I'm 5' 9" But have very short legs and long body and a 17" chair with a 5-6" footstool seems to be the magic formula. I also use memory foam which helps circulation and comfort.
One Ring to rule them all, One Ring to find them, One Ring to bring them all, and in the darkness bind them.