Based on the provided data and guidelines, my assessment is that Bouygues should be advised to issue **0%** of total adjusted capital in hybrid bonds over the next 18 months.

Here is the reasoning, following the provided guidelines:

*   **No Prior Hybrid Issuance:** The data explicitly states that Bouygues has never issued hybrid bonds. The guidance clearly notes that this is a "strong indicator" and a "reliable signal" that the entity has reasons not to do so.
*   **Funding Needs and Leverage:** While the company has experienced a significant increase in net debt (from €941m to €7,440m) and a deteriorating leverage trend, this is largely due to a major acquisition (€6,269m cash outflow for "Obtaining Control of Subsidiaries"). Its S&P Net Debt/EBITDA ratio of 3.26x is elevated but still within investment-grade territory for its sectors. There are no indications of "very high refinancing needs" or a "transformational capex/M&A program" on the horizon beyond what has already been financed.
*   **Cost of Debt:** The market data shows that the cost of debt has risen sharply (5-year swap rate moved from -0.264% in 2021 to 1.726% in 2022). The subordinated debt spread (Sub-Sen Delta) is around 200bps. Issuing a hybrid bond now would materially increase the company's current cost of debt compared to its existing senior borrowings, matching the description for the 0% option.
*   **Rating Profile:** Bouygues is a large, diversified industrial group. The massive increase in goodwill and debt is from the acquisition of Equans, which, while leveraged, is a strategic move now integrated into its financials. Its rating profile, while under pressure, is not described as having a "material downgrade risk without hybrid support" in its forward-looking outlook beyond the numbers already reported.
*   **Counter-Arguments for Higher Percentages:** The arguments for issuing hybrids (e.g., 7.5% or 11.25%) would require a scenario of "moderate refinancing or acquisition needs" or "rating headroom moderately constrained." While leverage is up, Bouygues has a long-term debt maturity profile and a history of managing its capital structure without hybrids. The one-time acquisition is already funded, and a new issuance program would be a permanent shift in financial policy, which is unlikely given its past stance.

**Final Answer:**

<answer>0%</answer>