Shattered Kingdoms

What is the save against summon?

My guess would be will. Certainly not reflex. Fortitude could be argued.

The real question is how the DC is calculated.

Think distance to target affects difficulty too

afaik, this is true, at least what others have told me in game. If you are in the same local (area like both inside Sith for example) it is much easier than if you were outside it. I don't know how far that extends, but there you go.

auto summon, willpower, and distance, also whether you are asleep or not.

Sleeping could just lower will saves across the board. We don't know.