Sorry for the late reply,
A lot of it depends on how flexible you are and your arm length. Ideally you would like your elbows cupped by your upper and lower legs inside your Indian style seated position.
If this position has me sighting in way low, I'll slide the butt of the rifle down in the pocket of my shoulder a bit or chalk up closer to the mag well with my off hand. Try to keep your off hand as far forward as you can though.
(I'm not able to grab the Key-Slots in the sitting position myself)