Let’s work through the reasoning step by step.

**1. Nature of Acciona’s business**  
Acciona is a Spanish infrastructure and renewable energy group, with activities spanning construction, energy (mainly renewables), water, and services. For credit analysis, the energy and infrastructure parts are the most relevant. The unregulated power and gas and transportation infrastructure methodology frameworks likely apply because a large share of EBITDA comes from renewables with long-term contracts (PPAs, CfDs) and concessions.

**2. Current balance sheet and leverage**  
We don’t have the exact adjusted debt and FFO figures, but we can infer:

- Equity attributable to owners of parent: €4,885m (2023-01-01)  
- Non-controlling interests: €1,419m  
- Total equity: €6,304m  
- Total assets: €22,595m  
- Debt-like items: non-current bonds/loans (€3,101m + €2,624m), current bonds (€1,139m), lease liabilities (€439m + €72m), etc.  
- Reported profit from continuing operations: €615m, operating profit: €1,334m.  
- Depreciation/amortization/provisions: €848m, so EBITDA is roughly €1,334m + €848m = ~€2,182m (plus some other adjustments).

This suggests FFO-to-debt is likely moderate for the current rating (BBB range), but the company is investing heavily (CAPEX of ~€2.2bn in 2022) and has significant growth ambitions.

**3. Funding needs and leverage pressure**  
- Investing cash outflows were €2.4bn vs. operating cash inflows €1.6bn → free cash flow is negative before disposals.  
- Net debt issuance (€835m net inflow from debt instruments) helped plug the gap.  
- The 2022 interest rate environment shifted dramatically (swap rates moved from negative to ~2%), which increases the cost of conventional debt.  
- The iBoxx subordinated delta of ~2.3% in 2022 vs. senior IG at ~1.1% means hybrid cost is higher but not dramatically so.

**4. Hybrid rationale**  
- Acciona already has hybrids in its capital structure (“Participaciones Preferentes Obligaciones Y Otros Valores Negociables” of more than €4.2bn total, both current and non-current). These likely already provide significant equity credit.  
- Given the existing large hybrid stock, adding more might not give proportional rating benefit—S&P caps equity credit at 15% of adjusted capital.  
- The company is growing rapidly but has moderate rating headroom; hybrid issuance could help maintain metrics while funding growth.  
- The cost of hybrid debt in 2022 is only modestly above senior debt (sub-sen spread ~1.2% over senior), so the penalty isn’t extreme.  
- However, with heavy existing hybrid usage, the incremental rating benefit per additional €1bn issued is lower; they may already be near the 15% cap.

**5. Mapping to guidelines**  
- The situation fits between “moderate funding needs / mild leverage optimization” and “moderate refinancing or acquisition needs / rating headroom moderately constrained”.  
- Given current high hybrid stock, a lower additional issuance percentage (3.75% of adjusted capital) is more plausible than 7.5% or higher—enough to maintain flexibility without hitting the cap inefficiently.  
- The cost of hybrid will increase the current cost of debt, but only modestly in 2022 terms.

**Conclusion:** A moderate additional hybrid issuance of around 3.75% of total adjusted capital is appropriate to support credit metrics and funding flexibility, without over-reliance or excessive cost.

<answer>3.75%</answer>