The sole reliance on free ...

The sole reliance on free chlorine is NOT good practice. As Marco pointed out that although the concept of free chlorine is simple, there are many factors which could lead to erroneous readings e.g. insufficient contact time, uneven distribution of dosed chlorine (short-circuiting), poor quality DPD tablets, water chemistry, etc. It is nice to understanding the theory of chlorine dosage - however, the challenges lie in its physical application, measurement and control. All factors must be considered including the quality of the measurement instruments, etc.