Let’s go step by step.

**1. Entity and sector context**  
TenneT Holding B.V. is a Dutch transmission system operator (TSO), i.e. a regulated utility, with the Dutch State as parent. Its core activity is electricity transmission — a natural monopoly under a well-established regulatory framework.

**2. Current capital structure and hybrid outstanding**  
At end-2022, Hybrid Capital = €2,125m.  
Equity attributable to owners of parent = €7,258m.  
Noncontrolling interests = €455m → total equity = €7,713m.  
Long-term borrowings = €19,006m; short-term borrowings = €709m; lease liabilities ≈ €729m total.  
Total adjusted capital (Equity + Debt, simplified) is approximately €28bn+.  
Outstanding hybrid is already ~7.5% of total adjusted capital if we use €28bn–€30bn as denominator.  
S&P’s 15% cap leaves room for additional issuance, but the starting point matters.

**3. Financial performance and metrics**  
Revenue grew significantly (€5,524m → €8,299m), but operating loss and net loss also widened.  
Profit/loss attributable to ordinary equity holders: –€967m in 2022 (–€401m in 2021).  
Finance costs rising (€202m → €300m), reflecting higher debt and rising rates.  
Cash flow from operations turned sharply down (€5,705m → €1,196m), partly from EEG working capital movements.  
Substantial capex: €4,424m in 2022 (vs €3,711m in 2021), likely to remain very high given energy transition.

**4. Leverage and headroom**  
Equity declined from comprehensive losses (–€726m in 2022), mitigated partly by €1,230m capital contribution.  
Debt increased sharply: long-term borrowings up ~€6.6bn.  
Credit metrics like FFO to debt are under significant pressure given operating losses and rising debt.  
The entity clearly requires substantial ongoing funding, large capex, and faces rating headroom constraints.

**5. Hybrid rationale**  
- Hybrids already form part of the capital structure, so market access is established.  
- Cost of hybrid vs senior debt: market data shows 2022 iBoxx € Non-Financial IG sub-sen delta ~1.2% above senior. That’s a moderate premium, not prohibitive for a highly rated TSO.  
- The rating profile is investment grade but under pressure from negative earnings, high capex, and growing leverage. Additional hybrid could support adjusted leverage metrics and preserve rating stability.  
- Given current outstanding hybrid (~7.5%–8% area), further issuance within the 15% cap can materially support credit metrics and funding flexibility.

**6. Mapping to the guidelines**  
- “High capex intensity” → fits TenneT.  
- “Significant leverage pressure” → yes, given losses and growing debt.  
- “Investment grade preservation strongly dependent on capital structure optimization” → moderately true; the equity injection by the Dutch State helps, but leverage pressure remains.  
- “Cost of hybrid will have a marginal impact on current cost of debt” — the delta is noticeable but relatively contained given the low absolute yield environment for high-grade issuers.  
- This points toward **at least 7.5% and possibly up to 11.25%** of total adjusted capital as appropriate.

Given the starting hybrid level, remaining capacity under the 15% cap, high capex needs, and rating headroom pressure, an advised additional issuance would bring total hybrid toward the upper-middle range — but not to the absolute 15% cap yet, absent a transformational M&A or extreme downgrade risk.

**7. Conclusion**  
An appropriate recommendation is **11.25%** of total adjusted capital, allowing meaningful additional issuance to support the balance sheet during a period of heavy investment and negative earnings, while staying within prudent limits.

<answer>11.25%</answer>