Terna exhibits solid scale with high regulatory framework typical of a regulated utility. In 2022, revenue rose to about €2.898B; Profit attributable to owners of the parent was €857M; equity surged to €6.149B with total assets around €22.8B. Leverage: long-term borrowings €8.416B (2023) vs equity €6.149B (2023), with total liabilities high; but hybrid bonds are already present: Equity Instruments Perpetual Hybrid Bonds with a face amount of €989M in 2022, and coupon payable already recorded. This indicates some hybrid capacity has been used and S&P-like rating agencies cap equity credit on hybrids at up to 15% of total adjusted capital (Equity + Adjusted Debt). We must consider remaining headroom. 2023 numbers show Equity Attributable To Owners Of Parent of €6.142B and Noncontrolling Interests €0.027B; Long-term borrowings €8.417B, Current portion €1.909B; total adjusted capital roughly Equity + adjusted debt (including hybrids). There is already €989M of hybrid in equity; the cap at 15% implies potential additional hybrids up to about 15% of total adjusted capital. Given 2022/2023 high capex needs (TERNA is a network operator with large investment cycle; we see property, plant, equipment grew to €16.2B; cash flows from operations robust but investing activities negative ~€1.855B in 2022), and the fact that hybrids are already used and there is high leverage pressure potential, a measured approach is prudent. The suggested options: 0%, 3.75%, 7.5%, 11.25%, 15%. Considering current hybrid outstanding of €989M and potential headroom, but to avoid overstating rating uplift, and given moderate to high capex needs, a mid-tier increment around 7.5% to 11.25% seems reasonable. The prompt asks to take into account starting point of outstanding hybrids and maximum per-year issuance up to €3B. I think headroom is moderate; to support leverage trajectory and capex, but not overdo cost of debt.

I will choose 11.25% as the extent of hybrid issuance to pursue over the next 18 months. It provides significant headroom to improve adjusted leverage while recognizing existing hybrid base and the need to preserve rating stability in a high-capex, capital-intensive regulated utility environment.

Final answer: <answer>11.25%</answer>