USUM is calculated from the 1-sigma components of displacement. It should be reasonably accurate because the calculation of USUM doesn’t dependent on the sign of the components. It is still somewhat conservative as it assumes the components are in-phase. A check on the calculation could be done in /POST26 by computing an RPSD of USUM for some location using a variable which was the SRSS combination of the component displacements. The square root of the integral of the USUM RPSD should match the /POST1 1-sigma value.