I don’t understand why you claim it would be impossible to reach the definition?
A simple constructed example: if the median income is € 20 by hour. A minimum wage of, say, € 15 would be considered safe. Why is that impossible?
A quick Google search shows me Destatis.de claims the median wage in April 2023 was € 24.59. Then 60% of that would be € 14.75. I would assume the median has increased a bit from 2023, and will increase further towards 2027, but I would from these numbers expect € 16 to for example be high enough to be considered safe from poverty, which isn’t that much higher than the proposed number.
The neat thing about using the median instead of the mean is that increasing the minimum wage only affects those earning the least, and thus doesn’t directly affect the median.
What you are referring to is absolute poverty. However, even if you can cover the very basics, such as rent, food and healthcare, if you have a lot less than most people that is known as relative poverty. Both are important to combat. I am assuming the definition is made as it is because it is quite simple to compute. Finding a good measure for cost of living is a lot more complex and easy to get stuck arguing what is an acceptable standard of living.