  We introduce a universal combination of susceptibility and correlation length
in the 3D Ising model, depending both on temperature and external magnetic
field. Starting from a parametric representation of the equation of state, we
study its behaviour close to the critical point. The results we derive can be
used as a sort of "reference frame" to chart the scaling region of the 3D Ising
universality class. More specifically, we focus on instances of Ising behaviour
in finite temperature QCD and, among these, we are particularly interested on
the critical ending point in the finite density, finite temperature QCD phase
diagram. In this context, Monte Carlo simulations are not possible and it is
particularly difficult to disentangle "magnetic-like" from "thermal-like"
observables, thus an explicit charting of the critical region could be useful
for a direct comparison of experimental results with QFT/Statmech predictions.
