This depends on many things: your height, the height of the chair, the height of the footstool, and ultimately, how much pressure you are putting on the foot. Find a chair that is the right height not to cut into the back of your legs. I find that having a cushion that tilts me slightly forward is very comfortable. Then adjust the footstool based on the chair's height. I'm 5'6" and find that chair that is somewhat lower than the standard dining room chair is most comfortable. Finally, be sure you're not bearing down on that leg while playing.
...plain Kate...and sometimes Kate the curst