To determine the appropriate level of hybrid bond issuance for TenneT Holding B.V. over the next 18 months, we must analyze the entity's financial profile, business risk, and market conditions based on the provided facts and S&P methodology.

1. **Business Risk Profile & Volatility**: TenneT is a transmission system operator (TSO) wholly owned by the Dutch State. According to S&P’s Regulated Utilities methodology, TSOs benefit from high regulatory advantage, low competitive risk, and very stable cash flows. Such entities typically qualify for the "low volatility" benchmark table. This strong intrinsic business profile allows them to sustain higher leverage (and conversely, benefit strongly from equity credit on hybrids) compared to more volatile corporates.

2. **Current Capital Structure & Hybrid Outstanding**: As of Jan 1, 2023, TenneT has €2.125 billion in Hybrid Capital already outstanding. Total Equity stands at €7.713 billion. Thus, hybrids already represent roughly 21.5% of the total equity and a significant portion of adjusted capital. S&P limits equity credit on hybrids to a maximum of 15% of total adjusted capital (Adjusted Debt + Equity). 

3. **Leverage & Financial Policy**: TenneT is undergoing a massive investment cycle, evidenced by a huge jump in Long-term Borrowings from €12.37 billion to €19 billion in 2022, and capex of €4.4 billion. Furthermore, the company reported a significant net loss (€-879 million in 2022) and suffers from negative retained earnings (-€967 million), meaning common equity is only sustained by continuous shareholder contributions (€1.23 billion in 2022) and hybrid capital. The FFO to Debt metrics are likely under severe pressure given the ballooning debt and negative net income. Thus, there is a strong fundamental need to optimize the capital structure and support investment-grade metrics.

4. **Market Conditions & Cost of Debt**: The 2022 swap curves show a sharp rise in interest rates (5Y swap at 1.726%, 10Y at 1.927%), and corporate bond spreads have widened (iBoxx EUR Non-Fin IG at 1.085% average, but 2.295% in 2022 for sub-senior). Issuing hybrids in this environment will materially increase TenneT's current blended cost of debt, which was historically anchored by lower rates. This cost penalty argues against maxing out the issuance capacity immediately.

5. **Target Issuance Level**: S&P’s cap for equity credit is 15% of total adjusted capital. If we assume a target adjusted capital base of roughly €25-30 billion (to cover the massive capex pipeline over the next 3 years), the 15% cap translates to roughly €3.75 to €4.5 billion in total hybrids. With €2.125 billion already outstanding, TenneT has room for approximately €1.6 to €2.3 billion in additional hybrids before hitting the 15% cap. Issuing roughly €1.5 to €2.0 billion over the next 18 months (well within the €3 billion/year limit) would bring the hybrid stack to about 11.25% of projected total adjusted capital. This level strongly corresponds to the criteria for 11.25%: "High capex intensity," "Significant leverage pressure," and "Investment grade preservation strongly dependent on capital structure optimization." Going all the way to 15% right now is unnecessary and would be penalized by the high marginal cost of capital in the current rate environment, but a mid-to-high range optimization is essential to stabilize the leverage trajectory during this transformational capex program.

<answer>11.25%</answer>