How you speculate about things you know nothing about is a far better gauge of problem solving skills than asking questions about an area where you can rely on learned facts, in my opinion.

It's not going to be accurate, but that is not the point. The point is whether you return a blank stare or start trying to reason and analyse the problem.

Sure, you might end up with any number of "I have no idea what the right value for this is - if it wasn't an interview, I'd look it up, but for the sake of argument let's say the value is X". That's fine. Your ability to make that leap and continue is more important than the right result (so is your ability to recognise and admit that you don't know and are substituting guesses)