Let’s reason through this step-by-step.

**1. Current capital structure**  
- Equity attributable to owners of parent (including hybrid) at end-2022: €7,258 mn  
- Hybrid capital: €2,125 mn  
- Total equity: €7,713 mn  
- Long-term borrowings: €19,006 mn  
- Total assets: €38,509 mn  

Hybrid capital is already ~29% of equity attributable to owners of parent and ~27.5% of total equity. Relative to total assets, hybrid is about 5.5%, and relative to long-term debt it’s about 11.2%.

**2. Profitability and coverage**  
- The company made a significant net loss in both 2022 and 2021.  
- Operating cash flows improved (€1,196 mn in 2022 vs €5,705 mn in 2021 due to working capital swings), but underlying operating cash flow excluding EEG working capital was only €596 mn.  
- Interest paid was €202 mn in 2022, finance costs €300 mn. Hybrid coupon payments were €57 mn each year.  
- The losses mean there is pressure on retained earnings and ordinary equity.

**3. Market environment**  
- In 2022, swap rates rose sharply (5Y: 1.726%, 10Y: 1.927%), meaning new fixed-rate debt would be more expensive.  
- Corporate bond yields also increased (iShares Core Euro Corp Bond yield rose to 1.085%, sub-senior non-financial IG yield to 2.295%).  
- Given the 0.2 delta for subordinated/senior spread, hybrid (deeply subordinated) would carry an even higher premium.  
- Rising rate environment makes additional hybrid issuance more costly relative to a few years ago.

**4. Rating and equity content**  
- Hybrid bonds usually get partial equity credit from rating agencies, typically up to a certain percentage of total capital.  
- Since TenneT is already loss-making, adding more hybrids would increase fixed coupon obligations, worsening interest coverage ratios and potentially pressuring ratings if the equity content limit is exceeded.  
- Regulated transmission system operators need stable credit ratings to finance large capex programs (capex was €4.4 bn in 2022).  

**5. Optimal reliance**  
Given the current ~27% hybrid-to-parent-equity ratio, the loss-making profile, rising interest costs, and the need to maintain strong credit metrics for heavy infrastructure investment, increasing reliance significantly would be imprudent. A moderate share around 25% seems plausible — aligning roughly with current levels — to keep some equity credit benefit without excessive fixed charges.

<answer>25%</answer>