To determine the optimal extent to which Naturgy Energy Group S.A. should utilize hybrid bonds over the next 18 months, we evaluate the company's financial risk profile, funding needs, leverage metrics, and the current cost of debt against the provided guidelines.

**1. Leverage and Rating Headroom:**
For the fiscal year ended December 31, 2022, Naturgy's EBITDA stood at €4.95 billion, a material increase from €3.53 billion in 2021.
Total Financial Debt (Long-term + Short-term borrowings and lease liabilities) was approximately €16.3 billion, while Cash and Cash Equivalents stood at an ample €3.98 billion. This leaves a Net Debt of roughly €12.3 billion. 
The resulting Net Debt-to-EBITDA ratio is about 2.5x, a substantial improvement from the prior year and exceptionally strong for a regulated/integrated utility in the "BBB" rating category. Because of this strong performance and cash flow generation, Naturgy currently enjoys ample rating headroom. Therefore, the company requires only a **mild leverage optimization rationale** and is not reliant on heavy hybrid issuance to defend its rating.

**2. Refinancing and Funding Needs:**
Naturgy's generated Operating Cash Flow of €4.24 billion easily covered its Capital Expenditures (€1.68 billion) and Dividend Payments (€1.5 billion), resulting in positive discretionary cash flow. Additionally, its robust cash position of €3.98 billion is more than sufficient to cover its current financial liabilities of €2.30 billion. Consequently, Naturgy’s refinancing and funding needs are **moderate to low**. 

**3. Cost of Debt vs. Cost of Hybrid:**
Based on the cash flow statement, Naturgy’s "Interest Paid" was €520 million on an average debt base of around €15.8 billion, giving an estimated current effective cost of debt of roughly 3.2% - 3.3%. 
Looking at the 2022 market metrics:
- 5Y Swap Curve: ~1.73%
- IG Corporate Spread: ~1.08%
- Sub-Senior Delta: ~2.29%
The expected yield on a new hybrid bond would be approximately 5.10%. Since 5.10% is noticeably higher than the company's average cost of debt (3.3%), issuing a new hybrid would **increase the current cost of debt**, making it less attractive to aggressively expand the hybrid layer beyond replacing existing calls.

**Conclusion:**
Naturgy possesses a very healthy balance sheet, comfortable liquidity, and reduced leverage following high profitability in 2022. While it maintains an existing hybrid layer as a standard financial tool for utilities, it has no fundamental need to stretch this mechanism (such as up to 7.5% or 11.25%) given its limited capital pressure and the increased cost of subordination. A relatively small allocation serves as optimal policy. 

<answer>3.75%</answer>