Mathematically stop points are meaningless. Think about it this way... If you are sitting at a poker table playing with world class pro's. after a short time you are kicking there butts even though you are outmatched you would probably want to get the hell outta there!... But if you are a world class poker player player with a bunch of slouches and you are getting your butt kicked you would want to keep playing because you just KNOW that things will turn around eventually.... Bottom line is if you have the advantage keep playing and never stop. If you dont get out of there and run!
Back to your question... me personally i dont ever like to leave down unless my trip BR has really tanked or it has just been a marathon of gambling not really getting anywhere. You should bring your whole BR with you when you go because you dont want to be in a situation that you are leaving because you lost too much, and can't afford to get it back. You should leave on a negative shoe, never leave on a positive shoe. Playing at those stakes you shouldnt have any heat so i wouldnt worry about keeping your sessions short. If you want to have some type of stop point... I would suggest maybe when you win $200 (way above ev) wait until the next negative count or the end of the shoe and then leave, but like i said in the long run it doesnt matter.