As others have already suggested, it depends on many factors. The most significant factors are the length of your torso, the height of your chair, and the length of your legs (especially from the floor to your knees). Personally, I prefer a chair that sits a little lower than average allowing the guitar to feel more secure. A good guideline for starting out is to get in a position where the neck of the guitar is not horizontal or vertical, but rather diagonal. When you are seated with good posture, the head of the guitar should be at about eye level.
It is common for those seeking a good default position to have the head of the guitar a bit too low. If the guitar is in a good position, then you should be able to maintain good posture, keep the thumb behind the neck, and play the highest 19th fret note on the first string with your 4th finger. This one task is very often a good indicator that you are in the ballpark. It is also common for students to believe they are in a good position, but then fail this little test of playing the highest note. When attempting this, it is very easy to raise the guitar a bit and find comfort and ease. The footstool is then raised accordingly.
Lastly, I am an advocate to alternatives to the footstool. I do believe it is good to first get maximum comfort with a footstool so that one has a good understanding of what they are trying to imitate. Along those lines though, I am all for footstool burning parties...LOL
Dr. Todd Tipton, Noda Guitar Studio
Charlotte, NC, USA (available via Skype)