Let’s reason through this step-by-step.

**1. Purpose of hybrid bonds / equity credit**  
Hybrid bonds receive partial equity credit from rating agencies (subject to caps like S&P’s 15% of total adjusted capital) because they absorb losses and have features like subordination and interest deferral. They help strengthen credit metrics without issuing common equity.

**2. Fortum’s current capital structure (end of 2022)**  
- Equity attributable to owners of parent: €7,670M  
- Noncontrolling interests: €67M  
- Total equity: €7,737M  
- Total liabilities: €15,905M  
So total adjusted capital (equity + debt) ≈ €23.6B. 15% of that = ~€3.5B cap for equity credit from hybrids.

**3. Leverage and interest coverage**  
From the P&L:  
- Comparable EBITDA 2022: €2,436M  
- Reported profit from continuing ops: €1,011M, but huge loss from discontinued ops.  
- Interest expense: €179M  
Interest coverage (Comparable EBITDA / Interest) ≈ 13.6x — quite strong.  
Overall debt is substantial vs. equity, but the coverage ratio demonstrates good capacity for additional subordinated instruments.

**4. Market conditions (interest rates)**  
- EUR swap rates (5Y, 7Y, 10Y) rose sharply in 2022 to ~1.7%–1.9% from negative/very low levels the prior years.  
- Corporate bond yields (iShares Euro Corp Bond) up, subordinated spreads wider (sub-sen delta ~2.3% in 2022).  
Rising rates mean hybrids are more expensive to issue now than in 2020–2021, but still possible if strategically needed.

**5. Why issue hybrids?**  
Fortum has had disrupted financials (large discontinued operations losses), a big drop in equity from the Uniper deconsolidation, and might want to strengthen the balance sheet without diluting shareholders. Hybrids would get some equity credit, supporting ratings if they are close to a downgrade threshold.

**6. How much to use?**  
Given decent coverage, the need for equity credit, but also high costs in the current rate environment, full usage (100%) of the 15% cap is unlikely — that would be ~€3.5B in hybrids, quite large relative to current equity. Aggressive but not impossible. A more moderate approach would be 50–75%, balancing credit enhancement and cost. Given the recent financial turmoil and already high leverage, leaning toward 50% is reasonable — they get benefit without overloading with costly subordinated debt.

**Final answer:**  
<answer>50%</answer>