My rule off thumb.Keep the top of your guitars body at your belly button(or an inch up above it,depending on the size of your torso,arm length and guitars body/shape.)
Its comfortable enough to not degrade picking and left hand use.If you have any pull or stretch in(or in back of)your right shoulder and/or the forarm at elbow length then you have it too low.The middle of your picking hand's palm should be able to rest on the low E comfortably.
Well thats more than rule of thumb.

Later! \m/