In this paper one introduces a bijective relation between the non–extensivity parameter q and the stationary probability density function f corresponding to an observable u for some super–additive systems for which the notion of Tsallis entropy applies. This relation is given as a comparison of linking functions characterizing memoryless of certain random variables associated to parameter q and the stationary density f. Then these results are used to formulate a method for approximating the parameter q based on an estimation of f, provided that the effective energy associated to u equals the effective kinetic energy. PACS: 05.20.Jj , 05.70.Ce MSC: 46N55, 28D20