To determine the appropriate extent to which TenneT Holding B.V. should be advised to issue hybrid bonds, we evaluate its financial profile, capital expenditures, and market conditions against S&P's guidelines.

**1. Capital Expenditure Intensity and Leverage Pressure:**
TenneT is a fully state-owned transmission system operator (TSO) tasked with massive grid investments critical for the energy transition. In 2022, the company exhibited incredibly high capex intensity, with capital expenditures on property, plant, and equipment reaching €4.42 billion—representing over 50% of its €8.30 billion in annual revenue. This extensive investment program has led to significant leverage pressure. Over the 2022 fiscal year, total borrowings ballooned from roughly €13.7 billion to €19.7 billion.

**2. Rating Preservation and Capital Structure Optimization:**
Preserving an investment-grade rating is strongly dependent on capital structure optimization given the high rate of debt accumulation. TenneT currently has €2.125 billion in hybrid capital, representing roughly 7.7% of its Total Adjusted Capital (TAC), which stands at approximately €27.4 billion (Total Equity of €7.71 billion + Total Borrowings of €19.71 billion). While there is significant leverage pressure, the presence of strong state support—evidenced by the €1.23 billion equity injection by the Dutch State in 2022—mitigates the "material downgrade risk without hybrid support" that would strictly necessitate pushing hybrid limits to the absolute 15% maximum. 

**3. Cost of Debt Considerations:**
Given the rapid increase in interest rates and swap curves throughout 2022 (e.g., the 10Y swap curve moved from an average of 0.053% in 2021 to 1.927% in 2022), the cost of issuing hybrids has risen. However, because TenneT is actively funding massive near-term projects with new debt at these prevailing rates, adding hybrids will still only have a marginal overall impact on their blended cost of debt while providing crucial equity credit (up to 50%) to stabilize adjusted leverage metrics. 

**Conclusion:**
TenneT fits the profile of a utility with high capex intensity and significant leverage pressure, relying on structure optimization to sustain its credit rating. However, the direct equity support from the Dutch state means it does not solely rely on hybrids to avert a downgrade. Advising an expansion to 11.25% of TAC would represent an issuance of roughly €1 billion in new hybrids, optimally balancing its moderate capacity, funding requirements, and the recent rate environment.

<answer>11.25%</answer>