The standard is a technical guide titled "Selection and dimensioning of high-voltage insulators intended for use in polluted conditions – Part 2: Glass and porcelain insulators." It provides specific rules for choosing and sizing ceramic and glass insulators based on the environmental pollution levels they will face. Key Content of IEC 60815-2
From Table 2 (Level d → 31–43 mm/kV). Choose 38 mm/kV as a conservative interim value.