Ferrovial’s balance sheet shows a large capital base (total assets €26.3 bn) with a significant debt load (adjusted debt ≈ €11.8 bn) and equity of €6.4 bn, giving adjusted capital of roughly €18.2 bn. The company is in a phase of high capex – its investments in infrastructure projects rose by about €2.5 bn in 2022 – and it faces a current‑year debt maturity of around €0.9 bn. Cash flow from operations (≈ €1.0 bn) is solid but not enough to cover the combination of the maturing debt and the planned expansion without increasing leverage.  

Under S&P’s methodology, hybrid bonds receive 50 % equity credit, so issuing hybrids improves the debt‑to‑equity ratio and moderates leverage metrics, which is crucial for preserving the investment‑grade rating. Given the company’s elevated leverage (net debt/EBITDA ≈ 9x), material capex requirements, and the need to refinance near‑term maturities, the optimal amount of hybrid financing is one that meaningfully reduces adjusted leverage while not causing a material increase in financing costs.  

The 11.25 % bucket (≈ €2.0 bn) aligns with “high capex intensity or large acquisition pipeline, significant leverage pressure, investment‑grade preservation strongly dependent on capital‑structure optimisation” and offers a marginal cost impact. This level provides enough hybrid capital to support the refinancing and investment programme while keeping the overall cost of debt only modestly higher.  

**Final answer:**  
<answer>11.25%</answer>