To determine the optimal extent to which Bouygues's capital structure should rely on hybrid bonds, we must evaluate the company's financial profile, credit standing, and the strategic role of hybrid securities.

1. **Financial Profile & Leverage**: Bouygues is a large-cap French industrial conglomerate with significant capital expenditures and acquisition activities (as evidenced by the massive increase in Goodwill and Cash Flows Used in Obtaining Control of Subsidiaries in 2022). By the end of 2022, its Net Debt surged to €7.44 billion from €0.94 billion the prior year, primarily driven by M&A. Shareholders' equity attributable to owners stands at €12.2 billion. The net debt to equity ratio is roughly 61%, and the total debt (long-term + short-term borrowings + overdrafts) to total capitalization is around 53%. This is a moderate to moderately high leverage level for an investment-grade BBB-rated industrial. 

2. **Credit Rating & Hybrid Treatment**: For a company like Bouygues, maintaining its investment-grade status (typically BBB/Baa) is critical for its construction and telecom operations. Hybrid bonds are treated as 50% equity by rating agencies (like S&P and Moody's) due to their subordination, deep subordination, and optional deferral of interest. Including hybrids in the capital structure allows the company to raise capital that economically acts like debt (tax-deductible coupons) while receiving equity credit to buffer its leverage ratios against rating downgrades during heavy investment or M&A phases.

3. **Industry Standards & Limits**: Rating agencies cap the amount of hybrid debt that can receive equity credit to prevent abuse, typically allowing up to 25% of the adjusted capitalization (equity + hybrids) to consist of hybrids. Exceeding this threshold results in the excess hybrids being treated as debt, nullifying the rating benefit. Therefore, companies targeting the optimal equity credit without wasting the instrument's structural advantages generally cap their hybrid reliance at exactly 25% of their total capital. 

4. **Optimal Weighting**: A 0% reliance ignores a valuable tool for managing rating-agency leverage metrics during expansive cycles. 50%, 75%, or 100% reliance would breach rating agency limits, causing the hybrids to lose their equity classification and severely hurting the company's credit profile and borrowing costs. Thus, the standard optimal reliance for a BBB-rated issuer using hybrids for partial equity credit is 25%.

<answer>25%</answer>