Average true flow stress-logarithmic true strain curves can be usually obtained from a tensile test. After the onset of necking, the average true flow stress-logarithmic true strain data from a tensile specimen with round cross section should be modified by using the correction formula proposed by Bridgman. But there have been no firmly established correction formulae applicable to a specimen with rectangular cross section. In this paper, a new easy-to-use formula is presented based on parametric finite element simulations. The new formula requires only incremental plastic strain and hardening exponents of the material, which are simply presented from a tensile test. The newly proposed formula is verified with experimental data for high strength steel DH32 used in the shipbuilding and offshore industry and is proved to be effective during the diffuse necking regime.