To determine the appropriate level of hybrid bond issuance for Bouygues, we must evaluate its financial position, leverage, refinancing needs, and the potential benefits of equity credit from hybrids under S&P Global Ratings' methodology.

**1. Current Financial Position and Leverage:**
As of December 31, 2022, Bouygues reports Total Equity of €13.93 billion and Total Debt (Long-term borrowings of €11.59 billion + Current borrowings/short-term of €1.36 billion + Overdrafts of €0.42 billion) of €13.37 billion. The reported Net Debt is €7.44 billion. 
However, S&P's adjusted debt typically includes lease liabilities. Adding non-current lease liabilities (€2.11 billion) and current lease liabilities (€0.50 billion) brings adjusted debt to approximately €15.98 billion. Total Adjusted Capital (Equity + Adjusted Debt) is thus around €29.91 billion. 
The FFO to Debt ratio (a key metric for S&P) would be under pressure. With operating profit of €1.87 billion and D&A of €2.67 billion, FFO is roughly €4.5 billion, yielding an FFO to adjusted debt of around 28%. This places the company in the "Significant" financial risk profile bracket (FFO/Debt of 20-30%), leaving limited headroom for an investment-grade buffer, particularly following the large acquisition in 2022 (Cash flows used in obtaining control of subsidiaries: €6.27 billion).

**2. Refinancing and Funding Needs:**
Bouygues has moderate to high refinancing needs, with €1.36 billion in current borrowings and portions of the €11.59 billion long-term debt maturing. Furthermore, the massive €6.27 billion cash outflow for acquisitions in 2022 demonstrates an active M&A pipeline (specifically the Equans acquisition) requiring funding and financial flexibility.

**3. Hybrid Bonds Rationale:**
Issuing hybrid bonds provides 50% equity credit under S&P criteria (up to the 15% cap of total adjusted capital). This meaningfully improves adjusted leverage metrics (lowering Debt/Capital and increasing FFO/Debt) without diluting shareholders. Given the recent M&A activity and the resulting leverage pressure, hybrid issuance is a strategic tool to optimize the capital structure and preserve the investment-grade rating. 

**4. Issuance Size Assessment:**
- At 3.75% of total adjusted capital (~€1.12 billion), the issuance provides limited but useful flexibility but may not sufficiently offset the leverage increase from the 2022 acquisitions.
- At 7.5% of total adjusted capital (~€2.24 billion, well within the €3 billion annual cap), the issuance would meaningfully improve adjusted leverage metrics. It supports the M&A funding requirement and restores rating headroom. The cost of the hybrid will slightly increase the current cost of debt (approx. 2.3% swap + 1.3% spread = ~3.6% yield), which is manageable for a BBB+ rated industrial.
- 11.25% or 15% would be excessive for Bouygues at this stage. While the acquisition was large, it is a single transformational event rather than an ongoing structural deterioration that would warrant maximizing the equity credit cap to stabilize a severe downgrade risk.

Therefore, an issuance targeting 7.5% of total adjusted capital optimally balances the need to restore leverage metrics post-acquisition while maintaining a prudent cost of capital.

<answer>7.5%</answer>