To determine the appropriate level of hybrid bond issuance for Terna S.p.A. over the next 18 months, we must evaluate the company's financial profile, credit metrics, existing capital structure, and market conditions.

1. **Existing Hybrid Framework & Starting Point**: 
Terna already has a significant amount of hybrid bonds classified as equity in its capital structure. As of Dec 31, 2022, the "Capital Instruments Bonds Hybrid Perpetual" stands at €989 million. S&P limits the equity credit given to hybrids to a maximum of 15% of total adjusted capital (Adjusted Debt + Equity). Issuing additional hybrids beyond this threshold would not receive equity treatment and would be rated as pure debt, defeating the purpose of optimizing leverage metrics.

2. **Credit Metrics & Adjusted Capital**:
- Reported Equity (2022): €6,169.1 million
- Reported Financial Debt (Long-term + Short-term + Current portion of LT - Cash): €8,416.7m + €444.1m + €1,909.3m - €2,155.1m = €8,615 million
- Existing Hybrids (treated as equity under S&P): €989 million
- **Total Adjusted Capital** = Adjusted Equity (€6,169.1m + €989m) + Adjusted Debt (€8,615m - €989m) = €14,784.1 million.

The S&P cap for equity-eligible hybrids is 15% of total adjusted capital:
15% × €14,784.1 million = **~€2.218 billion**.

Since Terna already has €989 million in outstanding hybrids, the **remaining capacity for equity-credited hybrids is approximately €1.229 billion** (roughly 8.3% of total adjusted capital). Issuing beyond this remaining capacity would exceed the 15% cap, meaning any further issuance would not improve adjusted leverage metrics.

3. **Business Profile & Leverage Rationale**:
Terna is a regulated transmission utility with a "Strong" regulatory advantage (typical for national TSOs in Europe under S&P methodology). The company has a solid investment-grade profile (typically 'A' range for such entities). Looking at the financials:
- FFO to Debt is healthy (FFO ~€1.9-2.0 billion vs. Debt of ~€7.6 billion adjusted, yielding ~25-26%, comfortably in the 'A' category). 
- There are no transformational M&A programs, and while capex is elevated (~€1.7 billion in 2022 for grid development), it is typical for a regulated TSO and largely pre-approved/tariff-supported, providing solid cost recovery visibility.
- Therefore, there is **no material downgrade risk** and no "significant leverage pressure" that would necessitate maxing out hybrid capacity (15%). 

4. **Market Conditions & Cost of Debt**:
In 2022, interest rates rose significantly (5Y swap moving from -0.26% in 2021 to +1.72% in 2022, with corporate spreads widening). Issuing long-dated subordinated hybrids in this rate environment would materially increase Terna's current cost of debt, especially since it is a refinancing/optimization exercise rather than a defensive necessity.

5. **Optimal Issuance Assessment**:
- 0% is too conservative; Terna already uses hybrids, and modestly utilizing remaining capacity (~3.75% of total adjusted capital = ~€553 million) optimizes the capital structure and provides slight rating headroom against capex intensity without pushing limits.
- 7.5% or higher would eat too much into the remaining capacity (approaching the hard 15% cap), which isn't warranted given the strong 'A'-range profile and lack of acute leverage stress. Furthermore, approaching the cap limits financial flexibility for future needs.

Thus, a limited issuance up to **3.75%** of total adjusted capital (roughly €500M-€550M, well within the ~€1.2B remaining capacity) appropriately balances mild leverage optimization with the higher cost of hybrid capital in the current interest rate environment, preserving the bulk of remaining capacity for future needs.

<answer>3.75%</answer>