To determine the extent to which FERROVIAL SA should utilize hybrid bonds relative to S&P Global Ratings' 15% cap on total adjusted capital, we compare the cost of hybrid bonds with the cost of equity. If the cost of hybrid bonds is lower than the cost of equity, it is optimal for the company to maximize its use of hybrid bonds up to the cap to minimize its overall weighted average cost of capital (WACC). 

1. **Cost of Debt**: 
   We estimate the firm's cost of debt by dividing the interest paid by the average total debt.
   Total Debt (2021) = 1,096,000,000 EUR + 9,513,000,000 EUR = 10,609,000,000 EUR
   Total Debt (2022) = 877,000,000 EUR + 10,776,000,000 EUR = 11,653,000,000 EUR
   Average Debt = (10,609,000,000 + 11,653,000,000) / 2 = 11,131,000,000 EUR
   Interest Paid (2022) = 329,000,000 EUR
   Cost of Debt = 329,000,000 / 11,131,000,000 ≈ 2.96%

2. **Cost of Hybrid Bonds**:
   Hybrid bonds are subordinated debt instruments and price at a premium over senior debt. Using the provided 2022 IBOXX Euro Non-Financial IG Sub-Senior Delta of 2.295%, we can estimate the cost of hybrid capital:
   Cost of Hybrid Bonds = Cost of Debt + Sub-Senior Delta = 2.96% + 2.295% = 5.255%
   (Furthermore, since hybrid interest payments are typically tax-deductible, the effective after-tax cost is even lower, making it highly attractive).

3. **Cost of Equity**:
   Using the Capital Asset Pricing Model (CAPM) with the 10-year swap rate as the risk-free rate (1.927% for 2022) and assuming a standard equity risk premium (e.g., 5-6%), the firm's cost of equity is noticeably higher than 5.255%. For infrastructure/construction groups like Ferrovial, the cost of equity easily sits in the 7% to 10% range.

Since the cost of hybrid bonds (which receive equity credit from rating agencies) is structurally lower than the cost of pure equity, substituting equity for hybrid bonds up to the permitted ceiling decreases the firm's WACC. Therefore, Ferrovial should fully maximize this advantage and utilize 100% of the allowable S&P capacity.

<answer>100%</answer>