**Reasoning**

1. **Current capital structure**  
   - Total equity (including non‑controlling interests) at 31 Dec 2022 = **€55.2 bn**.  
   - Adjusted debt (short‑term borrowings, current portion of long‑term debt, long‑term borrowings, current and non‑current lease liabilities) ≈ **€31.9 bn**.  
   - **Total adjusted capital** ≈ **€87.1 bn**.

2. **Existing hybrid instruments**  
   - From the equity‑component movements (perpetual subordinated bonds) the company already has about **€4.9 bn** of hybrid debt outstanding, i.e. roughly **5.6 %** of total adjusted capital.  
   - This leaves room to increase hybrid equity‑credit up to the S&P cap of **15 %** (≈ €13.1 bn).

3. **Funding environment**  
   - Capex has risen sharply (€7.7 bn in 2022 vs. €4.9 bn in 2021) and the company is pursuing a large energy‑transition investment programme that will continue into 2024.  
   - Refinancing needs are substantial: short‑term debt (≈ €7.5 bn) plus maturing long‑term debt must be rolled over in the next 18 months.  
   - Operating cash flow (≈ €17.5 bn) provides good coverage, but the combined effect of high capex and refinancing creates **moderate‑to‑high pressure on leverage** and on rating headroom.

4. **Rating considerations**  
   - ENI is investment‑grade (likely BBB+/A‑) but the rising debt‑to‑EBITDA and the need to fund a large acquisition/pipeline could erode that headroom.  
   - Hybrid instruments receive **equity credit** in S&P’s adjusted capital calculation; issuing hybrids therefore lowers adjusted‑debt ratios and helps preserve the rating.

5. **Cost of hybrid vs. benefit**  
   - Hybrid coupons are modestly higher than senior debt, but the **benefit of maintaining an “A‑” rating and avoiding a downgrade outweighs the marginal cost increase**.  
   - The company can comfortably absorb the additional coupon within its strong operating cash flow.

6. **Guideline thresholds**  
   - **High capex intensity or large acquisition pipeline** → 11.25 % category.  
   - **Significant leverage pressure** → same.  
   - **Investment‑grade preservation strongly dependent on capital‑structure optimisation** → also matches 11.25 %.

7. **Issuance limits**  
   - S&P permits a **maximum of €3 bn of hybrid per year**. Over 18 months the company could issue up to **€4.5 bn**.  
   - Adding €4.5 bn to the existing €4.9 bn would bring hybrid debt to ≈ €9.4 bn, i.e. **≈ 10.8 %** of total adjusted capital – very close to the 11.25 % band and well within the “high‑use” range.

**Conclusion**  
Given the elevated capex, refinancing needs, and the need to protect the investment‑grade rating, the company should **utilise hybrid bonds at the upper end of the allowed range, targeting roughly 11.25 % of total adjusted capital** (subject to the €3 bn‑per‑year issuance cap).

<answer>11.25%</answer>