**Reasoning**

1. **Total adjusted capital (debt + equity)**  
   - Debt: Long‑term borrowings + current borrowings = €28.083 B + €12.508 B = **€40.591 B**.  
   - Equity (total) = €39.285 B (including non‑controlling interests).  
   - **Total adjusted capital = €40.591 B + €39.285 B ≈ €79.876 B**.

2. **S&P “equity‑credit” cap**  
   - Maximum equity credit from hybrids = 15 % × €79.876 B ≈ **€11.98 B**.

3. **Current hybrid exposure**  
   - Deeply subordinated perpetual notes (treated as hybrid) = €3.393 B (2023).  
   - This provides equity credit of ≈ €3.393 B ≈ **4.3 %** of total adjusted capital, well below the 15 % cap.

4. **Leverage & financing context**  
   - Debt‑to‑capital ratio ≈ 50.8 % (high for a utility).  
   - 2022 swap‑curve rates turned positive (5 Y average = 1.73 %) making senior debt more expensive.  
   - iBoxx EUR non‑financial IG spreads rose (sub‑sen delta = 2.30 % in 2022), indicating tighter credit conditions.  

   → Hybrid instruments can lower reported leverage, improve credit metrics and reduce refinancing risk while still being treated partly as equity by S&P.

5. **Appropriate usage of the cap**  
   - Fully utilizing the 15 % cap (100 %) would be costly and signal over‑reliance on hybrid funding.  
   - A modest increase to **≈ 75 % of the cap** would bring the equity‑credit contribution up to roughly €9 B (≈ 11 % of capital), providing meaningful rating relief without excessive hybrid issuance.  
   - This level balances the benefit of equity credit against the higher coupon cost of hybrids and preserves financial flexibility.

**Conclusion:** ENGIE should increase its hybrid bond issuance to about **75 % of the S&P‑allowed equity‑credit cap**.

<answer>75%</answer>