To determine the appropriate extent of hybrid bond issuance for Eni S.p.A., we must evaluate the company's financial profile, credit metrics, existing hybrid structure, and the strategic rationale for issuance based on the provided guidelines.

**1. Current Financial Position and Leverage:**
As of December 31, 2022, Eni reports Total Equity of €55.23 billion and Total Financial Debt (Short-term borrowings + Current portion of long-term borrowings + Long-term borrowings) of €26.916 billion. 
- Adjusted Debt ≈ €26.9 billion
- Adjusted Capital (Debt + Equity) ≈ €82.2 billion
- Current Leverage (Debt / Capital) ≈ 32.7%

This is a very strong leverage profile for an Oil & Gas E&P major. The S&P methodology for E&P highlights that FOCF to debt and DCF to debt are key supplementary ratios. With an operating cash flow of €17.46 billion against €26.9 billion of debt, Eni exhibits robust cash generation relative to its debt load. 

**2. Existing Hybrids:**
The equity statement explicitly lists "Perpetual Subordinated Bonds" within equity. In 2020, Eni issued €3 billion of these bonds (net of €25 million issuance costs), and in 2021, another €2 billion (net of €15 million costs). While exact outstanding balances at year-end 2022 aren't explicitly totaled, the historical issuances and coupon payments of €138 million indicate a substantial existing hybrid portfolio (roughly €4-5 billion). This means Eni is already highly experienced with hybrids and has already utilized them to optimize its capital structure.

**3. Rationale for Issuance (0% to 15% guidelines):**
- **Credit Metrics & Rating:** Eni's 32.7% leverage and robust FOCF imply an "A" range standalone credit profile. There is no material downgrade risk, and the company does not face significant leverage pressure that would necessitate hybrid support to preserve its investment grade rating. 
- **Funding Needs:** While E&P companies always have maintenance and development capex (Eni spent €7.7 billion on PP&E in 2022), this is well-covered by operating cash flows. There is no indication of "transformational capex/M&A" or "very high refinancing needs" that would push the rationale toward the 11.25% or 15% thresholds.
- **Cost of Debt:** The 5Y swap rate in 2022 rose to 1.726%, and corporate IG spreads (Sub-Sen) widened to 2.295%, meaning a new hybrid coupon would likely be in the 5.5%-6.5% range. This would represent a material increase over Eni's historical cost of debt, which benefits from its strong IG rating and legacy issuance.
- **Hybrid Utility:** Issuing a small amount (3.75% of adjusted capital ≈ €3 billion) provides limited but useful rating flexibility, allowing Eni to maintain its strong metrics while funding ongoing capex or potential tuck-in acquisitions without slightly deteriorating its leverage. However, given that Eni already has significant hybrids in its stack, the marginal benefit of adding more is reduced, and the cost increment is less justifiable.

Because Eni's fundamental credit metrics are strong, it has low refinancing risk, its capex is easily covered by internal cash flow, and the cost of incremental hybrid debt would be relatively high in the current rate environment, there is no compelling need to issue hybrids to defend its rating or fund operations. Thus, 0% is the most prudent advice.

<answer>0%</answer>