Have a look at the curriculum for the local university. It will give you an idea of what level to expect.
> but I also don't want to sign off on someone who can't write a for loop.
So you've got a lower bound now. You can either start from there and get more advanced with every step, or think of some level you find appropriate and write both easier and harder questions. Then move higher/lower as you get more information from the candidate.
I don't think "find all the phone numbers in this file" is a bad question. You can scale it really well from "show me how you read a file and split words" to "let's talk about running parallel tokenizers over mmaped pages". There may be a better example for the domain you work in though.