Hello,
for example in ratio model - there are actually two zscores the model works with: zscore_bid and zscore_ask
the zscore_bid is calculated from the bid1/ask2 ratio, the zscore_ask is calculated from the ask1/bid2 ratio
then for instance to enter the long pair position, the zscore_ask must cross the entry threshold and to exit the position, the zscore_bid must cross the exit threshold
inverse for the short pair position
I hope that helps...
Karel