This paper concentrates on an extension of a rather new methodology to determine local residual gradients at an enhanced lateral resolution using the so-called slit milling technique. The method is based on stress relaxation by making a slit using a focused ion beam and the displacements are measured through digital image correlation so as to calculate the residual stress. Our novel approach consists of a multiple fitting procedure along the length of the slit instead of the commonly applied averaging method. The applicability of our approach is demonstrated when stress gradients exist. In accordance to the Saint-Venant principle in linear elasticity, longer slits are better than shorter slits because of end effects. The proof-of-principle is supported by measurements on steel under controlled bending and by finite element modeling.