Based on the financial data and the S&P rating methodology provided, here is the assessment for NATURGY ENERGY GROUP SA's optimal use of hybrid bonds over the next 18 months:

1. **Track Record and Sector Context**: 
Naturgy is a large, integrated European utility (Regulated Utilities / Unregulated Power & Gas). Companies in this highly capital-intensive sector frequently utilize hybrid bonds to optimize their capital structure and fund long-term assets without diluting shareholders or stretching senior debt metrics. Naturgy has a well-established track record in this market, having first issued hybrid bonds in 2013 and remained active with issuances in 2021/2022. This demonstrates management's commitment to using hybrids as a permanent structural layer of capital. 

2. **Current Financial Health and Leverage Metrics**: 
At the end of fiscal year 2022, Naturgy's credit metrics were robust. The company reported a strong S&P FFO / Net Debt ratio of 28.47% and a low S&P Net Debt / EBITDA ratio of 2.67x. Operating cash flows surged to EUR 4.24 billion in 2022 (up from EUR 1.0 billion in 2021) driven by favorable energy market dynamics. Consequently, Moody's noted an "Improving" adjusted leverage trend. 

3. **Total Adjusted Capital Analysis**: 
The company holds approximately EUR 10 billion in equity and EUR 14.8 billion in financial debt, bringing total adjusted capital to roughly EUR 24.8 billion. A 7.5% hybrid allocation would equate to an outstanding hybrid stock of approximately EUR 1.8 to 1.9 billion. This perfectly aligns with the standard size of hybrid portfolios maintained by solid investment-grade utility issuers of Naturgy's scale. 

4. **Category Selection**:
Because Naturgy’s leverage trend is improving and its core ratios (such as FFO/Net Debt) are healthy, the company is not facing "significant leverage pressure" or "material downgrade risk." Therefore, pushing hybrid issuance to the maximum limits of 11.25% or 15% is unnecessary. However, allocating 0% or a minimal 3.75% would run contrary to their recent issuance activities and the ongoing utility capex cycles (which require moderate but sustained funding). An allocation of 7.5% is the most appropriate target, providing a balanced approach to satisfy moderate ongoing refinancing needs, preserve rating headroom, and meaningfully optimize leverage metrics against a backdrop of increasing interest costs.

<answer>7.5%</answer>