Based on the financial facts provided for TERNA S.p.A. for the fiscal year ended December 31, 2022, we can evaluate the company's capital structure and its utility of hybrid bonds:

1. **Capital Structure & Leverage:** 
   Terna reported total equity of approximately €6.17 billion. Its gross debt is approximately €10.77 billion (comprising €8.42 billion in long-term borrowings, €1.91 billion in the current portion of long-term borrowings, and €0.44 billion in short-term borrowings). After deducting cash and cash equivalents of €2.16 billion, net debt stands at roughly €8.61 billion. Thus, Terna's total adjusted capital (Equity + Net Debt) is approximately €14.78 billion. 

2. **Current Hybrid Bond Issuance:**
   The financial statements show that Terna issued its first perpetual hybrid bonds in 2022, amounting to €989 million. As a percentage of total adjusted capital, this outstanding hybrid debt represents about 6.7% (€989 million / €14.78 billion). 

3. **Credit Metrics and Rationale:**
   Terna operates in the regulated utilities sector (electricity transmission), which generally benefits from high predictability of cash flows and regulatory advantages. The S&P Net Debt to EBITDA ratio stands at a manageable 4.35x, and the S&P FFO / Net Debt is 17.24%. Furthermore, Moody's indicates an "Improving" adjusted leverage trend. Because Terna's credit metrics are relatively strong and improving, the company is not under immense pressure to aggressively optimize its capital structure to preserve its investment grade rating (which would otherwise warrant pushing hybrid bonds towards the 11.25% or 15% thresholds). 

4. **Conclusion:**
   The fact that Terna actively issued €989 million in hybrid bonds in 2022 signals a clear intent to maintain moderate funding flexibility and a mild-to-moderate leverage optimization rationale to support its capital expenditure program (investing in the national electricity grid) without diluting its strong rating profile. Staying close to its established ~6.7% proportion points squarely to the 7.5% bucket as the most reasonable and strategically aligned limit for hybrid bonds.

<answer>7.5%</answer>