Single-passage NMRON results for dilute ¹²⁵Sb in single-crystal Fe have been acquired as a function of RF power for various applied DC fields along the (100) 'easy' axis and quantitatively analysed. This is the first time that such data have been described in terms of an absolute theory without the benefit of arbitrary ordinate scaling factors. The introduction of a simple model to account for the observed dependence of the RF skin depth on applied DC field is shown to be successful in two ways. First, it enables the determination of the relaxation constant, C_K=1.0 s K (0 T), 2.25 s K (0.3 T), 3.0 s K (0.45 T) and 3.25 s K (0.6 T), and secondly it permits sensible estimates of the adiabatic parameter at the surface, A_s, to be deduced.