To determine the advisable extent to which ENI S.P.A. should utilize hybrid bonds over the next 18 months, we must consider both the company's historical capital structure and its current financial position. 

1. **Previous Hybrid Issuance**: ENI has an established track record of issuing hybrid bonds, having inaugurated its hybrid program in 2020 with €3 billion, followed by an additional €2 billion in 2021. No new hybrid issues were recorded in 2022. This brings the outstanding perpetual subordinated bonds to €5 billion. 

2. **Total Adjusted Capital (TAC)**: Based on the balance sheet for the end of 2022, ENI's total equity is €55.2 billion. Its reported debt (short-term and long-term borrowings, plus lease liabilities) is approximately €31.8 billion, meaning its TAC is roughly €87 billion. The €5 billion in existing hybrids represents approximately 5.7% to 6.5% of its TAC (depending on net versus gross debt adjustments). This falls squarely within the intermediate 7.5% framework. 

3. **Current Leverage and Credit Metrics**: ENI's credit metrics improved significantly in 2022, benefiting from strong commodity prices and robust operating cash flows. The company’s Net Debt / EBITDA ratio stands at a very healthy 0.95x, and its FFO / Net Debt is strong at 65.93%. Moody’s also reported an improving leverage trend. Given these exceptional metrics, ENI's rating headroom is substantial. 

4. **Strategic Rationale**: While ENI does not currently face large, unmanageable refinancing or transformational M&A pressures that would justify maxing out its hybrid capacity to 11.25% or 15%, the company has structural incentives to maintain its existing hybrid base. Maintaining an intermediate allocation optimizes its weighted average cost of capital (WACC) and provides useful rating flexibility (a "cushion") against the inherent cyclicality of the oil & gas exploration and production sector, as well as to fund energy transition capex.

Given that the current stock of hybrids is clustered tightly around the middle of the allowable scale and considering that the cost of hybrid debt would only slightly increase its current average cost of debt while preserving its strong investment-grade cushion, ENI should be advised to maintain a moderate bucket of hybrids. 

<answer>7.5%</answer>