To estimate **FFO / Net debt** using an S&P-style approach:

**1. Estimate FFO**

A practical approximation of S&P Funds From Operations is:

\[
\text{FFO} \approx \text{Cash flow from operations before working capital} - \text{cash taxes paid} - \text{net interest paid}
\]

From the 2022 cash flow data:

- Cash flows from operations before changes in working capital: **€4,804.3m**
- Income taxes paid: **€557.4m**
- Interest paid: **€637.7m**
- Interest received / operating asset interest: **€78.5m**

Net interest paid:

\[
637.7 - 78.5 = 559.2
\]

So:

\[
\text{FFO} \approx 4,804.3 - 557.4 - 559.2 = 3,687.7 \text{ million}
\]

**2. Estimate S&P-adjusted net debt**

Use gross financial debt, including lease liabilities and concession liabilities, less cash and cash equivalents.

Debt components at 2023-01-01:

- Noncurrent financial liabilities excluding concession liabilities: **€19,692.1m**
- Current financial liabilities excluding concession liabilities: **€6,521.4m**
- Noncurrent lease liabilities: **€1,656.2m**
- Current lease liabilities: **€496.5m**
- Noncurrent concession liabilities: **€1,680.5m**
- Current concession liabilities: **€243.2m**
- Bank overdrafts and other cash position items: **€213.6m**

Gross debt:

\[
19,692.1 + 6,521.4 + 1,656.2 + 496.5 + 1,680.5 + 243.2 + 213.6 = 30,503.5 \text{ million}
\]

Less cash and cash equivalents:

- Cash and cash equivalents: **€9,012.2m**

\[
\text{Net debt} \approx 30,503.5 - 9,012.2 = 21,491.3 \text{ million}
\]

**3. FFO / Net debt**

\[
\frac{3,687.7}{21,491.3} = 0.1716
\]

So the estimated **FFO / Net debt ratio** is approximately **17.2%**.

<answer>17.2%</answer>