Your knees should be literally just under the edge of the keyboard. (As a general rule.) And you should really ideally be sitting on the front half of the stool with your feet flat on the floor slightly (or slightly more than slightly!!

) in front of your knees....... if that helps at all....

Oops - sorry - read your question wrong........

Generally you should be sitting at a height where your arms are parallel to the floor........so no, there is no fixed x cm distance as such......it depends on the person......