Bow Flex ties should be proof-tested using a Target Load Test Unit. The actual tensile loading required for stabilising a bowed wall is usually supplied by the specifying engineer.
It is not uncommon to find that this loading is surprisingly low. For example, considering a wall of 5m of height and bowing outwards at its mid point by 50mm, it is straightforward to calculate, by using a triangle of forces, that a horizontal load of 1kN is sufficient to withstand a vertical load of 50kN or 5 tonnes! With Bow Flex ties fixed at 600mm centres a load resistance in excess of 80kN per metre run is easily achieved.