okay - looks live you have determined the resistance point (which looks like it is at its highest deviation above the mean) to jump in the trade and looking for it to mean revert (which is why you say as long as it goes down you will make money). By choosing to enter at this point, the likelihood of mean reversion is high.
Otherwise the perception may be that if there is any little deviation between the pairs you have chosen you can jump in as "spreads are low risk".
My personal exit criteria is when that ratio hits its mean.
Also with your graph it is spanning over years, how long do you plan to stay in this trade??