For dance, apparently a sit position in a dance spin is considered a "difficult" position if the thigh is parallel to the ice.
(See pictures 18 vs. 19 in http://www.usfigureskating.org/conte...C_05-08.29.pdf ).
For freestyle, maybe that should be considered the minimum just to qualify as a sit position (or variation, e.g., broken leg or pancake) when a sit position is required, otherwise it's considered an upright spin. But that's pretty harsh at the lower levels, or in back sitspins, where positions like picture 18 are pretty common.. Better just to consider it as deserving -1 GOE.
Then a variation where the skater goes all the way down, such as sitting on the back of the other boot, could count as a difficult position or +1 GOE.