To determine the appropriate extent of hybrid bond utilization for REN - Redes Energéticas Nacionais, SGPS, S.A., we must evaluate its financial position based on the provided 2022 data and the S&P industry criteria for Regulated Utilities.

**1. Financial and Risk Assessment:**
*   **Liquidity and Capital Position:** As of year-end 2022, REN has total Equity of approximately €1.518 billion and total Borrowings (long-term + current portion) of approximately €2.334 billion.
*   **Capex and Regulatory Environment:** As a regulated utility, REN operates under a cost-plus framework, which typically allows for the recovery of capital investments. However, the company is dealing with significant changes, notably the *Asset Related To The Transitional Gas Price Stabilization Regime*, which reflects a substantial, non-recurring balance sheet entry (roughly €1 billion) that impacts liquidity and capital structure requirements.
*   **Refinancing and Leverage:** The company faces a dynamic interest rate environment (swap rates for 7Y and 10Y rose significantly in 2022 to ~1.8-1.9%). While the company’s operating cash flow remains robust (€613 million from operating activities), the increase in debt service costs and the need to maintain a strong rating profile for capital-intensive infrastructure operations suggest that leverage management is a priority.
*   **Hybrid Rationale:** The entity’s current capital structure shows significant borrowing relative to equity. Using hybrid bonds can offer "rating headroom" by providing equity-like characteristics (50% equity credit per S&P methodology) without diluting existing shareholders. 

**2. Choosing the Optimal Level:**
*   **0% to 3.75%:** Seems insufficient given the company's capital-intensive nature and the need to manage debt ratios in a rising interest rate environment.
*   **7.5%:** This aligns with a scenario where the company needs moderate leverage optimization to maintain its credit rating. Given the inflationary pressures on interest rates and the size of the *Transitional Gas Price Stabilization Regime* liability (which creates a temporary "lock" on working capital), REN would benefit from the rating headroom that a 7.5% allocation provides. This allows for a meaningful improvement in adjusted leverage metrics without the excessive cost burden of a larger hybrid issuance (11.25% or 15%), which might be disproportionate to the current refinancing needs.

**Conclusion:** A 7.5% utilization strikes a balance between preserving rating headroom, managing moderate refinancing pressure, and avoiding the higher costs associated with aggressive hybrid issuance.

<answer>7.5%</answer>