Rare are times you get to know the exact exits. This setup requires an estimate extension & an exit plan to be pre-determined in-case of over-extension. Sometimes things go wrong and it over-shoots, hence the 'maximum impossible' extension should be put in mind. The 'maximum impossible extension' can only be predetermined by countless hours of constant monitoring of the pairs. The factored that to lot sizing. One Mistake that is common on such setups is pilling /martingale /compounding/ averaging. This setup goes with 'sit and wait for exit'. I don't know if any of this makes sense, hope it does. I'm just sharing my thoughts.