Johnson Cook model - flow stress match with UTS at ductility?

[GMarsh]

Hi, I am trying to use Johnson Cook model for predicting equivalent flow stress. Just to check its validity at extremities, I input ductility as plastic strain, but I don't see it output stress close to ultimate tensile strength. I used ductility over 1 millisecond to calculate plastic strain rate. All other parameters like A, B, C, n - I am using as given in standard available literature. Is it not necessary for equivalent flow stress to match with ultimate tensile strength at plastic strain matching with ductility?

Thanks,
Geoff