**Reasoning**

1. **Total adjusted capital (debt + equity)**  
   – Total assets (2023‑01‑01) = 388 132 M € (≈ 388 bn €).  
   – This figure equals the sum of total debt plus total equity, i.e. the adjusted capital base for the S&P cap.

2. **S&P’s 15 % equity‑credit ceiling**  
   – 15 % × 388 bn € ≈ 58.2 bn € of equity credit may be attributed to hybrid instruments.

3. **Current hybrid exposure**  
   – Interest paid on perpetual sub‑ordinated bonds (2022) = 606 M €.  
   – Assuming a market‑typical coupon of ≈ 5 % for hybrids, the outstanding hybrid principal is roughly 12 bn €.  
   – With S&P’s usual 50 % equity‑credit treatment, the current equity‑credit provision is about **6 bn €**, far below the 58 bn € ceiling.

4. **Financing needs and leverage**  
   – The group generated a negative operating cash flow (‑7.4 bn €) and has massive capex (≈ 18 bn €).  
   – Debt‑to‑assets is ~ 88 % (341 bn € debt vs. 388 bn € assets) and equity has shrunk from 62 bn € (2021) to 46.6 bn € (2022).  
   – To improve credit metrics and retain an investment‑grade rating, EDF must reduce reported leverage.

5. **Benefit of hybrid issuance**  
   – Hybrid bonds receive equity credit, converting part of the debt into equity for rating purposes, thereby lowering the adjusted‑debt ratio.  
   – The cost of hybrid financing is generally lower than senior debt because of the equity‑credit benefit, while still being higher than plain equity; it therefore offers an attractive funding source.

6. **Utilising the cap**  
   – With ample headroom (≈ 52 bn € of equity credit still available) and a clear need to deleverage, the most efficient strategy is to issue hybrids up to the maximum allowed equity‑credit level (i.e. 100 % of the S&P cap).  
   – This would add roughly 52 bn € of hybrid principal (assuming 50 % equity credit), improving the adjusted‑debt ratio from ~ 88 % to about 73 % and supporting the rating.

7. **Conclusion**  
   – EDF should make full use of the permitted 15 % equity‑credit ceiling for hybrid instruments. The optimal extent of hybrid‑bond usage, relative to the cap, is therefore **100 %**.

<answer>100%</answer>