To determine the optimal utilization of hybrid bonds relative to S&P Global Ratings' 15% cap on equity credit, we must analyze the company's capital structure and leverage profile. 

At the end of 2022 (2023-01-01), ITALGAS S.P.A.'s capitalization components were as follows:
- **Short-Term Financial Debt**: 142,437,000 EUR
- **Long-Term Financial Debt**: 6,402,913,000 EUR
- **Total Debt**: 6,545,350,000 EUR
- **Total Equity**: 2,390,570,000 EUR
- **Total Adjusted Capital (Debt + Equity)**: 8,935,920,000 EUR

The company's leverage ratio (Debt / Total Adjusted Capital) is highly elevated at approximately 73.2% (6,545 / 8,935). With such a high debt burden, preserving credit ratings is of paramount importance to ensure access to debt capital markets at reasonable interest rates. 

Since hybrid bonds offer 50% equity credit from S&P (up to the 15% of total adjusted capital limit) without diluting existing shareholders, a highly levered infrastructure/utility company like Italgas should maximize its hybrid capacity. Issuing hybrids up to the maximum allowable cap provides critical rating support and optimizes the weighted average cost of capital (WACC) compared to issuing pure equity.

Therefore, the company should utilize 100% of its available hybrid bond capacity under the S&P limits.

<answer>100%</answer>