To determine the appropriate extent to which ENGIE should utilize hybrid bonds over the next 18 months, we must examine the company's current capital structure, leverage metrics, and historical use of hybrid capital.

1. **Current Hybrid Utilization**: According to the balance sheet data provided, ENGIE's "Deeply Subordinated Perpetual Notes" stand at 3.393 billion EUR at the end of 2022 (down from 3.767 billion EUR in 2021). 
2. **Total Adjusted Capital (TAC) Estimation**:
   - Total Equity is reported at approximately 39.28 billion EUR.
   - Total Borrowings (Long-term + Current) amount to roughly 40.59 billion EUR, and Cash and Cash Equivalents are at 15.57 billion EUR, yielding a Net Debt of approximately 25 billion EUR.
   - Depending on whether TAC is estimated on a gross or net debt basis (roughly 64 billion to 80 billion EUR), ENGIE's current hybrid capitalization accounts for approximately 4.2% to 5.2% of its TAC.
3. **Credit Metrics and Leverage Profile**: ENGIE has an improving adjusted leverage trend. The S&P Net Debt to EBITDA ratio stands at a healthy 3.25x and S&P FFO / Net Debt is at an ample 24.83%. This signals strong credit performance and comfortable rating headroom, negating the need for high levels of equity support from hybrids to defend its investment-grade rating.
4. **Conclusion on Hybrid Need**: Since ENGIE possesses solid liquidity, a robust rating profile, and is demonstrating an improving leverage trajectory, it has a mild leverage optimization rationale rather than an urgent requirement to shore up metrics or finance a massive transformational acquisition/capex program. The existing stock of deeply subordinated notes already provides a limited but useful cushion of rating flexibility without materializing outsized hybrid refinancing costs in a rising yield environment. 

Given these considerations, an optimal and historically consistent layer of hybrid bonds is squarely aligned with a 3.75% target of Total Adjusted Capital.

<answer>3.75%</answer>