To determine the extent to which ITALGAS S.P.A.'s capital structure should rely on hybrid bonds, we must compare the relative costs of different financing instruments and evaluate the company's financial position. 

1. **Cost of Debt Analysis**: 
   The market data provided includes a "SUB-SEN DELTA FOR IBOXX EUR NON-FINANCIAL IG", which tracks the yield premium (spread) of subordinated/hybrid debt over senior investment-grade debt. In 2022, the average delta is 2.295%. This indicates that subordinated hybrid bonds are significantly more expensive to issue than standard senior debt. 

2. **Capital Structure Optimization**:
   Generally, a company seeks to minimize its Weighted Average Cost of Capital (WACC). Because hybrid bonds are subordinate to senior debt and carry equity-like risks for investors, they demand a higher yield. Unless a company is facing strict borrowing constraints, requires an emergency infusion of capital without diluting shareholders, or needs to aggressively defend a deteriorating credit rating (as rating agencies often treat a portion of hybrids as equity), pure senior debt is the preferred, cost-effective debt instrument.

3. **Company Financials**:
   Looking at ITALGAS S.P.A.'s balance sheet at the end of 2022 (going into 2023), the company maintains robust operating cash flows ("Cash Flows From Used In Operations" of ~548 million EUR) and growing revenues. Additionally, its "Long Term Financial Liabilities Excluding Other Non Current Financial Liabilities" stand at roughly 6.4 billion EUR, while "Other Noncurrent Financial Liabilities" (where hybrids might traditionally be grouped if not under standard debt) are negligible at just 34,000 EUR. The company operates successfully with a traditional debt and equity mix, indicating no structural or distressed need for hybrid financing.

Because senior debt is substantially cheaper and the firm faces no explicit rating constraints that mandate the use of equity-credited debt instruments, the optimal allocation of hybrid bonds in the capital structure to minimize capital costs is zero.

<answer>0%</answer>