As others have said, whatever is comfortable. At this stage, it's probably best to find something comfortable, and (more importantly) keep it consistent. When you've figured out how to fly the thing, then experiment with different heights.
As an example, in a 172, I have the seat all the way up. I can't land or do manoeuvres if I can't see where I'm going. In my current airplane (Airbus) I sit higher for takeoff, and lower for landings. My previous jet, I sat high all the time.
Until you find out what works for sure, comfort and consistency are key.