TenneT shows substantial capital needs and high leverage pressures, with long-term borrowings of 19.0b and hybrid capital at 2.125b as of 2023-01-01. EBITDA/cash flow metrics are negative for period, and large capex plans likely exist given utility transmission infrastructure. The hybrid cap provides equity credit up to 15% of total adjusted capital, and given current structure (existing hybrid 2.125b out of total adjusted capital roughly equity 7.258b plus debt etc), there is room for optimization. The company is regulated transmission infra with potentially steady regulatory cash flows, but likely needs to strengthen balance sheet to maintain investment-grade profile amidst high capex. The S&P framework implies hybrids can bolster leverage metrics with limited rating impact if used prudently, especially with moderate to high capex. Considering the maximum annual issuance cap of 3b and target cap 15%, a measured approach could be beneficial. Given "moderate refinancing or acquisition needs" and to maintain rating headroom, a 7.5% allocation (i.e., issuing up to 7.5% of total adjusted capital over 18 months) seems to provide meaningful leverage improvement and rating flexibility with manageable cost impact.

Final: <answer>7.5%</answer>