Based on the provided financial and market data for TenneT Holding B.V., the following observations support the recommendation for the extent of hybrid bond issuance:

1. **Current Hybrid Capitalization**: TenneT's hybrid capital stood at 2.125 billion EUR on January 1, 2023. At the same time, Total Equity was approximately 7.71 billion EUR, and Total Debt (including long-term and short-term borrowings, plus lease liabilities) was roughly 20.4 billion EUR. This brings the Total Adjusted Capital to approximately 28.1 billion EUR. The current level of hybrid bonds thus represents almost exactly **7.5%** of the company's total adjusted capital. As guidelines note, an entity's previous level of hybrid issuance is a strong indicator of a reasonably advisable level for the next 18 months.

2. **Leverage and Capital Expenditure Pressures**: The company exhibits significant capex intensity, with expenditures for property, plant, and equipment reaching over 4.4 billion EUR in 2022 to support the energy transition. Additionally, operating profitability was squeezed (Profit Loss Before Tax: -1.23 billion EUR), leading to extreme leverage ratios for 2022 (S&P Net Debt / EBITDA ratio of 83.85 and Moody's signaling a deteriorating leverage trend).

3. **State Support vs. Capital Optimization**: Under normal circumstances, such extreme leverage pressure combined with transformational capex might push a corporate entity closer to an 11.25% or 15% hybrid threshold to preserve an investment-grade rating. However, TenneT is 100% owned by the Dutch State, which injected 1.23 billion EUR in standard equity during the year ("Increase Decrease Through Other Contributions By Owners"). TenneT benefits heavily from Sovereign support, which buffers rating downgrade risks and reduces its absolute dependency on maxing out hybrid capitalization limits.

4. **Strategic Role of Hybrids**: Keeping the issuance level at roughly 7.5% provides TenneT with a moderate but meaningful improvement to its adjusted leverage metrics, easing off pure debt accumulation without drastically increasing the blended cost of financing during a period where market swap rates have turned substantially higher (10Y Swap curve jumped to 1.927% in 2022). 

Therefore, advising TenneT to maintain its hybrid bond issuance around the 7.5% mark properly addresses its moderately constrained rating headroom and ongoing funding needs while factoring in its substantial government-backed equity support.

<answer>7.5%</answer>