I often see a recipe that require something to be boiled in "lightly salted water" e.g. Semmelknödel, but I'm always in doubt on how much salt that actually is.
Can anyone tell the "correct" ratio of salt and water for lightly salted water?
There is no such correct ratio. Just go with whatever feels lightly salted to your personal taste. The point is simply to differentiate from recipes in which people go for lots of salt, such as some noodle cooking techniques which require the water to be "as salty as seawater" or similar.
In the end, every cook has their own subjective scale of what is their "typical" amount of salt, and it is impossible to meet the recipe author's without further information. And even if you happen to meet it, if your personal preferred level is ckose, you won't notice much difference. On the other hand, if your preference is very different, the food won't taste right to you if you use the same absolute ratio of salt to water. So, the only practical thing is to use your own taste as a reference.