Based on the analysis of ACEA S.P.A.'s financial data and market conditions, the company exhibits high capex intensity (capex of €1.05 billion against operating cash flow of €0.73 billion) and significant leverage pressure (debt/EBITDA of approximately 4.45x and FFO/debt of 13.6%). The negative free cash flow after dividends (approximately -€470 million) indicates substantial external funding needs. Additionally, the company is pursuing acquisitions and faces refinancing requirements. Issuing hybrid bonds at 11.25% of total adjusted capital (approximately €0.91 billion) would provide meaningful improvement to adjusted leverage metrics while preserving the investment-grade rating, with only a marginal impact on the cost of debt. This aligns with the guideline for 11.25% when high capex intensity and significant leverage pressure are present, and investment-grade preservation strongly depends on capital structure optimization.

<answer>11.25%</answer>