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.
The effects of using a bit more or less salt in the cooking water should be hardly noticeable, so it should be rather hard to make a mistake here unless you are using extrem amounts.
The goal of using salted water is to avoid too much salt dissipates from the food into the water. So if you want to take things over the top you could calculate the percentage of salt in your Knödel and adjust the water to it so food and water are sharing the same osmotic pressure to maintain the Knödels salt content exactly and unchanged.