  In this article, the importance is demonstrated of a proper choice of
reference particles for decay angle definitions, when constructing partial-wave
amplitude of multi-body decays using helicity formalism. This issue is often
ignored in the standard use case of the helicity formalism. A new technique is
proposed to determine the correct particle ordering, and can also be used as a
generalized method to calculate the rotation operators used for the final-state
alignment between different decay chains. Numerical validations are also
performed to support the arguments and new technique proposed in this article.
