To determine the optimal extent to which Fortum Oyj should utilize hybrid bonds relative to the S&P Global Ratings' 15% cap on total adjusted capital, we evaluate the company's capital structure and the macroeconomic cost of capital.

1. **Analysis of Capital Structure**:
At the end of 2022, Fortum's Total Debt was approximately €7.78 billion (Noncurrent Financial Liabilities of €3.66B + Current Financial Liabilities of €4.13B). With Cash and Cash Equivalents standing at €3.92 billion, the company holds a positive net debt position of around €3.86 billion. Its total equity is roughly €7.74 billion. This robust, capital-intensive structure (Total Adjusted Capital ≈ €15.5 billion) indicates a structural need for long-term financing where maintaining investment-grade credit metrics is crucial.

2. **Cost of Capital (WACC) Optimization**:
Using the provided 2022 market data:
*   **10Y Swap Curve (Average)**: 1.927%
*   **iShares Core Euro Corp Bond Spread**: 1.085%
*   **Sub-Sen Delta (for subordinated/hybrid debt)**: 2.295%

The approximate yield on a newly issued hybrid bond would be:
`1.927% (Risk-Free/Swap) + 1.085% (Senior IG Spread) + 2.295% (Subordination Premium) = 5.307%`

For a major utility like Fortum, the cost of equity generally ranges between 7% and 10%. Because the cost of subordinated hybrid debt (approx. 5.3%) is significantly lower than the cost of equity—and because hybrid coupon payments are frequently tax-deductible—utilizing hybrid bonds heavily suppresses the Weighted Average Cost of Capital (WACC). 

3. **Ratings Agency Benefit**: 
S&P Global Ratings provides 50% equity credit for hybrid bonds up to a limit of 15% of total adjusted capital. Maximizing this allowance allows a company to essentially borrow at debt-like costs while fortifying credit-rating equity metrics, shielding it from rating downgrades that would occur if standard senior debt were utilized instead.

Since Fortum operates with significant leverage and positive net debt, mathematically optimizing WACC dictates taking full advantage of the cheaper equity credit offered by hybrid bonds up to the maximum allowable threshold. 

<answer>100%</answer>