The scope of this note is a self-contained presentation of a mathematical method that enables us to give an absolute upper bound for the difference of the Gini coefficients $$\left|G({\sigma }_1,\cdots ,{\sigma }_n)-G({\gamma }_1,\cdots ,{\gamma }_n)\right|,$$ where $({\gamma }_1,\cdots ,{\gamma }_n)$ represents the vector of the gross wages and $({\sigma }_1,\cdots ,{\sigma }_n)$ represents the vector of the corresponding super-gross wages that is used in the Czech Republic for calculating the net wage. Since (as of June 2019) ${\sigma }_i=100·⌈1.34{\gamma }_i/100⌉$, the study of the above difference seems to be somewhat inaccessible for many economists. However, our estimate based...