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Inspired by recent multiscale modeling of void/microvoid interaction in ductilefracture, a new analysis of literature data on ultrahigh strength martensitic steelsshows that JIC toughness normalized by primary inclusion mean spacing isparabolically related to the measured critical primary void size ratio. Interpretingthis critical ratio as a measure of the critical intervoid ligament strain forprimary void coalescence by finer scale microvoid instability, calculation of thecritical strain from Rice-Tracey void growth shows that JIC toughness isexponentially related to the critical microvoid instability strain. This represents afar stronger sensitivity to critical strain than the previously assumed linearscaling. An approximate correlation of this critical strain to macroscopic fractureductility allows an estimate of fracture toughness.