In order to build a reliable constitutive model for magnetic shape memory alloys (MSMAs) the availability of accurate experimental data for calibration and validation purposes is essential. However, the demagnetization effect and the resulting sample shape-dependent difference between the applied field and the internal field makes measurements of MSMAs properties difficult to interpret. Since for non-ellipsoidal specimen the internal magnetic field and thus the induced magnetization is nonuniform, standard demagnetization factors can not be applied without evaluation of the expected error. Following up on previous work by the authors this paper describes a methodology by which experimental data can be interpreted more accurately. The procedure involves the numerical solving of nonlinear magnetostatic boundary value problems for MSMAs in which the stress-dependent magnetic properties of the material are predicted by the constitutive model. New results of this analysis presented here include a parametric study of the sample shape dependence of the demagnetization effect when accounting for the nonlinear magnetization properties of MSMAs.