How much is "a ladleful"?
Every sourdough starter has different characteristics, a ladleful of one will give a different result from the same amount of another. The recipe is right in that there's no way to say what result you are going to get, so why be exact. 

The point I would make is that you want repeatibility, and the capability to adjust your amounts over time to suit your taste. So whatever you use to get it out (ladle, spoon, measuring cup, hand) I would weigh and record how much starter you use, you can add more or less and see how you get. As long as you keep using the same starter you can adjust to your taste.