To determine the appropriate extent to which Terna S.p.A. should utilize hybrid bonds, we evaluate its financial position, regulatory framework, capital structure, and S&P’s methodology guidelines. 

**1. Capital Structure and Adjusted Leverage Metrics:**
Based on the 2022 annual report, Terna S.p.A. has total equity of approximately €6.17 billion and gross debt of around €10.77 billion (long-term borrowings of €8.42B + current portion of €1.91B + short-term of €0.44B). Deducting its cash and cash equivalents of €2.16 billion yields a net debt of about €8.61 billion. 
S&P's Total Adjusted Capital (TAC) can be estimated as Equity + Net Debt = €6.17B + €8.61B = €14.78 billion. 
In 2022, Terna issued its first perpetual green hybrid bond amounting to €989 million (recognized in equity). 
Currently, hybrids represent about **6.7%** (€989M / €14.78B) of its Total Adjusted Capital. This closely aligns with a 7.5% threshold.

**2. Capex Intensity and Rating Headroom:**
As the Italian electricity transmission system operator, Terna is a highly regulated utility ("low volatility") with predictable cash flows (FFO over €2 billion). However, Terna is undergoing a significant capital expenditure program to support Italy's energy transition (capex is highly intensive, shown by €1.49 billion invested in PP&E in 2022). While Terna operates with a strong, solid investment-grade rating (BBB+/A- range), this massive and persistent capex pipeline moderately constrains rating headroom. The issuance of the hybrid helps to protect this rating by taking advantage of S&P's 50% equity credit, which meaningfully improves adjusted leverage metrics (like FFO-to-Debt) without diluting shareholders.

**3. Cost of Debt:**
Terna's legacy cost of debt is very low (finance costs of €121.8M on €10.77B of debt is an average cost of slightly over 1.1%). An issuance of a hybrid bond typically carries a higher yield due to subordination and perpetuity (in 2022, Terna's hybrid coupon was fixed at 2.375%). Adding this to the capital structure only *slightly increases* the aggregate cost of debt while defending the credit rating, which falls perfectly in line with the 7.5% guideline parameters. 

**Conclusion:**
With a 6.7% current hybrid-to-TAC allocation, moderately constrained rating headroom due to strong transition capex needs, a meaningful improvement in adjusted leverage from the issuance, and only a slight increase to its exceptionally low historical cost of debt, Terna is best positioned in the 7.5% category. 

<answer>7.5%</answer>