We derive entropy formulas for finite reservoir systems, Sq, from universal thermostat independence and obtain the functional form of the corresponding generalized entropy-probability relation. Our result interprets thermodynamically the subsystem temperature, T1, and the index q in terms of the temperature, T , entropy, S , and heat capacity, C of the reservoir as \documentclass[12pt]{minimal}
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\begin{document}$ T_1=T \exp(-S/C)$\end{document} and \documentclass[12pt]{minimal}
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\begin{document}$ q=1-1/C$\end{document} . In the infinite C limit, irrespective of the value of S , the Boltzmann-Gibbs approach is fully recovered. We apply this framework for the experimental determination of the original temperature of a finite thermostat, T , from the analysis of hadron spectra produced in high-energy collisions, by analyzing frequently considered simple models of the quark-gluon plasma.