This article digs into the numeric uncertainty for supraharmonic assessment methods and proposes a robust reference for calibration. A detailed resource survey of candidate methods, digital Special International Committee on Radio Interference (D-CISPR) and light-quasi-peak (QP), shows that computing resources should not be the primary concern, considering both necessitates an acceptable level of resources and requires the same mid-range field-programmable gate array (FPGA). Furthermore, a new criterion for selecting and optimizing numerical methods is proposed: the variability caused by time-shifting operations. D-CISPR, a proposed D-CISPR variation, the light-QP, and the proposed numerical-heterodyne methods were studied with the CISPR 16 impulse train test and three examples of real-world signals. Results demonstrate that the variability concerning time shift is less than 1% for the D-CISPR-based methods but up to 50% for the light-QP method.