A magnetic nanostructure array was fabricated by post-patterning of a L10 ordered 5-nm-thick FePtCu film revealing a rather broad switching field distribution (SFD). The individual contributions to the SFD of the dot array were investigated by micromagnetic simulations. Based on transmission electron microscopy results, the dots show a truncated cone shape which was directly used for the finite element model. The influence of single parameters, i.e., easy axis distribution, magnetic anisotropy, and dot size, on the SFD was estimated quantitatively and compared. Furthermore, the influence of damage induced during the nanofabrication process was analyzed and correlated with experimental results.