To determine the appropriate level of hybrid bond issuance for Acciona SA, we must evaluate its financial profile, credit metrics, and market conditions based on the provided data.

1. **Credit Metrics and Leverage Position**: 
   - Reported Debt (Noncurrent + Current loans and lease liabilities): €3,688 million.
   - Reported Equity: €6,304 million.
   - EBITDA (estimated from Operating Profit + D&A): €1,334m + €848m = €2,182 million.
   - Reported FFO to Debt is relatively healthy, but Acciona operates in the unregulated power and gas sector, which is characterized by moderately high cyclicality and significant capital intensity. The company has massive capex requirements (€2,195m in 2022) to fund its renewable expansion, which significantly pressures free cash flow and pushes leverage metrics higher.

2. **Current Hybrid Outstanding**: 
   - Under "Participaciones Preferentes Obligaciones YOtros Valores Negociables No Corrientes1" (a typical Spanish GAAP line for perpetual subordinated instruments/hybrids), Acciona already has €3,101 million outstanding. This is a substantial existing hybrid base, representing roughly 15-20% of its adjusted capital pre-adjustments, indicating the company is already deeply utilizing hybrid capital to manage its leverage.

3. **S&P Cap Consideration**:
   - S&P limits equity credit for hybrids to a maximum of 15% of total adjusted capital (Adjusted Debt + Equity). Given that Acciona already has €3.101 billion in hybrids, any new issuance must be carefully weighed against this hard cap. Adding more would risk breaching the 15% threshold, which could result in S&P stripping the equity credit from the new or even existing hybrids, rendering the issuance ineffective for leverage optimization.

4. **Market Conditions and Cost of Debt**:
   - 2022 market data shows a sharply rising interest rate environment (5Y swap moving from -0.264% in 2021 to 1.726% in 2022, with sub-senior spreads widening to 2.295%). Issuing hybrids in this environment would dramatically increase the company's cost of debt, as hybrids require a significant premium over senior bonds.

5. **Strategic Rationale**:
   - Acciona has high capex intensity, which typically supports a higher hybrid usage (11.25% - 15%). However, because it already carries a massive €3.1 billion hybrid stack, its capacity to add more under S&P's 15% cap is practically exhausted. Therefore, further issuance provides marginal rating flexibility but poses a high risk of violating the equity content cap. A limited, modest issuance (3.75% of total adjusted capital) could optimally top off the cap space without overshooting it, providing slight buffer for the massive capex pipeline while acknowledging that the cost of debt will increase.

Given the existing €3.101 billion in hybrids already pushing the limits of the 15% S&P cap, any new issuance should be highly constrained. A 3.75% allocation allows for minor leverage optimization to support the high capex intensity while respecting the cap boundaries and avoiding an unsustainable cost of debt increase in the 2022 rate environment.

<answer>3.75%</answer>