To determine the optimal extent to which Veolia Environnement should utilize hybrid bonds over the next 18 months, we evaluate the company's financial profile, strategic position, and recent historical reliance on these instruments.

1. **Capital Structure and Hybrid Base**: 
Based on the fiscal year ending December 31, 2022, Veolia holds roughly €3.5 billion in deeply subordinated securities (hybrid bonds). Evaluating this against the total adjusted capital (Total Equity of ~€14.9 billion + Gross Debt of ~€30.3 billion = ~€45.2 billion), the outstanding hybrid stack represents approximately 7.7% of its total adjusted capital. This serves as a strong empirical indicator of the company’s structural target for hybrid capitalization. 

2. **Leverage and Rating Headroom**:
Veolia's S&P Net Debt / EBITDA ratio stood at a relatively elevated 3.92x in 2022 following the transformational acquisition of Suez, leading to a "deteriorating" leverage trend as noted by Moody's. While the heaviest phase of the M&A pipeline is now complete (shifting to integration and realization of synergies), rating headroom remains moderately constrained. Hybrids are playing a meaningful role in providing equity credit and improving leverage metrics to defend its investment-grade rating without diluting shareholders.

3. **Refinancing and Strategic Needs**:
Because the large Suez M&A has already materialized, the entity currently has moderate refinancing and ongoing operational needs rather than an immediate transformational M&A program requiring maximum (15%) hybrid support. The existing debt and hybrid maturity profile demands proactive but stabilized refinancing.

4. **Cost of Capital**:
Although interest rates have spiked (e.g., the 5Y swap curve averaged 1.726% in 2022 compared to negative yields in previous years) and hybrid coupons will marginally increase the overall cost of debt, this slight increase in funding cost is a fair trade-off for Veolia to maintain capital flexibility and preserve its credit rating.

Given the existing stock of hybrids hovering around the 7.7% mark and an easing (but still moderately constrained) post-acquisition leverage profile, the company comfortably aligns with the characteristics of the 7.5% threshold.

<answer>7.5%</answer>