Based on the financial data and market context provided for REDEIA CORPORACION SA (formerly Red Eléctrica Corporación, S.A.), the following analysis supports a recommendation for a low but non-zero utilization of hybrid bonds (around 3.75% of total adjusted capital) over the next 18 months:

1. **Robust Credit Metrics & Mild Leverage Optimization:** 
Redeia operates in the highly predictable Regulated Utilities sector (electricity transmission), yielding very stable and resilient cash flows. For FY 2022, the company demonstrated a very strong S&P Net Debt to EBITDA ratio of 3.21x and an FFO to Net Debt of 20.63%, which are firmly aligned with strong investment-grade parameters for this industry. Furthermore, Moody's indicated an improving adjusted leverage trend. Because the company's baseline metrics are already solid, it is not facing severe leverage pressure; therefore, any hybrid issuance would be driven by a "mild leverage optimization rationale" rather than a critical need to preserve an investment-grade rating. 

2. **Moderate Funding Needs & Sufficient Cash Flow Coverage:**
In 2022, Redeia generated strong operating cash flows of €1.56 billion, which easily covered its capital expenditures (approximately €536 million) and dividend distributions (approximately €544 million). Because the company generates comfortable free cash flow and has no transformational M&A requirements outlined, its refinancing and new funding needs are inherently moderate. 

3. **Hybrid Bond Precedents and Proportions:**
The data explicitly indicates that 2023 is the first year of hybrid bond issuance for the company. Estimating the total adjusted capital (Total Equity of ~€4.89 billion + Total Borrowings of ~€6.21 billion = ~€11.1 billion), a moderate hybrid issuance (e.g., a standard €500 million green hybrid benchmark transaction) mathematically represents roughly 4.5% of total adjusted capital. This perfectly aligns with the 3.75% conceptual bucket representing limited but useful rating flexibility.

4. **Cost of Debt Impact:**
Given Redeia's very low embedded cost of debt (its finance costs are roughly €116 million on average debt of €6.5 billion to €7 billion, yielding a ~1.7% cost of debt), the pricing for a subordinated hybrid bond in the 2022/2023 market environment (incorporating higher swap rates and sub-senior spreads) would be notably higher. Consequently, utilizing hybrid bonds will increase the current cost of debt, which disincentivizes stretching the hybrid bucket up to the 7.5% or 15% thresholds absent any large acquisition pipeline. 

In conclusion, a limited hybrid bucket acts as an optimal, supplementary buffer for mild capital structure optimization without excessively burdening the company's cost of capital.

<answer>3.75%</answer>