Ferromagnetism in certain B2 ordered alloys such as Fe60Al40 can be switched on, and tuned, via antisite disordering of the atomic arrangement. The disordering is accompanied by a ∼1 % increase in the lattice parameter. Here we performed a systematic disordering of B2 Fe60Al40 thin films, and obtained correlations between the order parameter (S), lattice parameter (a 0), and the induced saturation magnetization (M s). As the lattice is gradually disordered, a critical point occurs at 1-S = 0.6 and a 0 = 2.91 Å, where a sharp increase of the M s is observed. DFT calculations suggest that below the critical point the system magnetically behaves as it would still be fully ordered, whereas above, it is largely the increase of a 0 in the disordered state that determines the M s. The insights obtained here can be useful for achieving tailored magnetic properties in alloys through disordering.