To determine the optimal extent to which EDP, S.A. should issue hybrid bonds over the next 18 months, we evaluate the company's financial profile, capital requirements, and historical issuance behavior against the provided guidelines.

1. **Capex Intensity and Funding Needs**: 
   EDP demonstrates high capital expenditure requirements, typical of leading European utilities heavily engaged in the energy transition. In 2022, cash payments relating to property, plant, and equipment and intangible assets exceeded €3.49 billion. This aggressive investment pipeline is primarily driven by its expansive renewable energy projects (e.g., through its subsidiary EDP Renováveis), matching the criteria of "High capex intensity." 

2. **Leverage and Capital Structure Optimization**: 
   The company's S&P Net Debt to EBITDA ratio stands at 4.06x, with an FFO to Net Debt ratio of 18.84%. While these metrics are somewhat characteristic of the regulated utilities and unregulated power sectors, they still imply significant leverage pressure. At these levels, capital structure optimization—such as the utilization of hybrid bonds—is highly critical to preserving the company's investment-grade rating and supporting its continuous growth agenda without excessively straining its senior debt metrics. 

3. **Hybrid Issuance History and Rating Momentum**:
   EDP is a sophisticated, recurring issuer of hybrid bonds, with a track record extending back to 2013 and recent issuances in 2021/2022. This signals that hybrid instruments are already a structural component of its financial strategy. Furthermore, Moody's indicated an "Improving" adjusted leverage trend for 2022. Because its leverage is stabilizing/improving, EDP is not facing the "material downgrade risk" that would force it to max out at the 15% cap under distressed conditions. Instead, a strong yet sustainable reliance on hybrids fits perfectly with maintaining rating flexibility.

4. **Interest Rate Environment**:
   Given the macroeconomic shift in swap curves and the iBoxx EUR Non-Financial IG yields rising through 2022, the cost of issuing new hybrids has increased but remains a justifiable expense for EDP relative to its cost of equity, having a marginal impact on its overall weighted cost of capital while supplying essential 50% equity credit from rating agencies.

In conclusion, EDP fits the profile of a heavy capex spender relying significantly on capital structure optimization to safely anchor its investment-grade rating, pointing to a hybrid capacity allowance on the higher end, but comfortably below the absolute maximum cap.

<answer>11.25%</answer>