when were these books published? cus if I understand my own research correctly, the casino rules are getting tougher. I have been told to leave at anything less than 0 or even +1. I guess it depends on the size of your br, what your bet spread is like, and how much you're willing to risk.
I wouldnt base any of these decisions on the level of the count though. I would consider more specific things like Betting Correlation, Playing Efficiency and other ratings. you can find a comparison here:
http://www.qfit.com/card-counting.htm
if I understand correctly, good PE is more advantageous in small deck games (which is what Zen is mostly designed for) and good BC is more advantageous in larger deck games.