Let’s work through this step by step.

**1. Entity profile**
- Terna S.p.A. is Italy’s national electricity transmission system operator, a regulated utility with a natural monopoly.
- Revenue is largely from regulated activities and related-party transactions with CDP Reti.
- Profitability is strong and stable: operating profit €1,333m (2022); net profit €857m.
- Large asset base (€17.7bn noncurrent assets), heavy capex (€1.49bn in 2022).
- It issued its first perpetual hybrid bond in 2022 (€989m), classified in equity.

**2. Regulatory advantage**
- Italy’s regulatory framework for Terna is transparent, provides cost recovery, and includes pass-through mechanisms for major investments.
- Regulatory stability and independence are reasonable for the sector, though Italy’s sovereign risk adds some constraints.
- The final regulatory advantage would likely be **Strong/Adequate** or **Adequate**, given the framework but acknowledging country risk.

**3. Financial risk profile and credit metrics**
- S&P metrics show Net Debt/EBITDA of 4.35x and FFO/Net Debt of ~17.2% in 2022.
- Moody’s leverage trend is improving.
- These levels suggest an investment-grade profile but with moderately constrained headroom, especially given high ongoing capex.
- 2022 hybrid issuance of €989m already partly optimizes the capital structure.
- Given the large capex program, some pressure on leverage metrics persists, but not extreme.

**4. Market conditions**
- By 2022, swap rates had risen sharply (5Y ~1.73%, 10Y ~1.93%), and corporate bond yields were higher.
- Subordinated spreads widened somewhat but remained manageable.
- Cost of hybrid issuance had increased versus 2020–2021, but not dramatically relative to senior debt.

**5. Guideline mapping**
- The company already has hybrid bonds outstanding (issued in 2022), so "no current hybrid" does not apply.
- It has moderate-to-high capex, mild-to-moderate rating headroom constraints, and hybrid issuance provides useful flexibility.
- A further hybrid issuance would slightly increase weighted average cost of debt but would help preserve metrics amid high investment.
- The guidance for **7.5%** fits: “Moderate refinancing or acquisition needs; rating headroom moderately constrained; hybrid issuance meaningfully improves adjusted leverage metrics; cost of hybrid will slightly increase the current cost of debt.”
- Since 2022 hybrid issuance was ~€989m and equity is ~€6.17bn, the 2022 ratio was already around 15% of equity, but as a percentage of total adjusted capital (equity + adjusted debt), it would be lower. Another incremental issuance within the next 18 months would likely bring it toward the 7.5% zone rather than the max 15%.

**6. Conclusion**
Given the existing hybrid, continued high capex, moderate rating headroom, and slightly higher funding costs, a **7.5% of total adjusted capital** is a reasonably advisable next step.

<answer>7.5%</answer>