For high voltage impulse testing, a standard lightning impulse is defined in IEEE Std. 4 and IEC 60060-1 as a double exponential waveform having a front time T1 = 1.2 μs ± 30% and time to half-value T2 = 50 μs ± 20%. It has been noticed that for a given specimen, it is possible to successfully pass a flashover test at one end of the T1 tolerance range while failing the same test at the opposite end of the tolerance spectrum. Consequently, a systematic approach was adopted to investigate this observation. Up-and-down tests were performed to define the disruptive discharge voltage (critical flashover voltage CFO, U50) for 1, 5, 10, and 15 unit glass insulator strings standard lightning impulses using the minimum acceptance front time value (T1 ≈ 0.84 μs). Tests were repeated using the maximum tolerance value (T1 ≈ 1.56 μs) to investigate the degree of divergence in the flashover value. Particular attention is given to the steepness (voltage-time characteristics) of the applied impulse to consider if tolerance criteria amendment is merited in a future standard revision. As the steepness impact is more renowned in non-uniform geometries, field homogeneity as a function of string length is also incorporated into the analysis.