For most players, there are four points of contact between the body and the banjo pot: two where the banjo rest on the legs, one where it rests against the stomach, and the last where the forearm rest against the arm rest (or rim). The last one is the critical one; make sure you keep enough pressure against the armrest to keep the banjo neck supported without the help of the left hand. I usually make my new students remove their left hand completely away from the neck two or three times while playing a piece to make sure the neck stays in place.
Once you're comfortable with supporting the neck with the right forearm, you can then concentrate on left hand positioning.
My advise to my banjo students is to keep the left hand as relaxed as possible. The thumb can extend a little above the neck until a long stretch of the left hand fingers is required (for instance when stretching the pinky to the fourth or fifth fret of the fourth string). When executing a "long stretch" with the left hand, drop the thumb down a little on the back of the neck and place it opposite the middle and ring fingers. A good way to get the hang of this is to remove the hand from the neck and touch the thumb to the middle and pinky, then place the hand back on the neck trying to retain the same positioning.
Once again, let the requirements of the finger stretches of the left hand determine when to drop into the "classical guitar" position; keep the hand relaxed and comfortable otherwise.
Sorry if I've rambled, but this is my approach to the left hand...
Thanks... Mike