When designing a stabilator (sp?), where should the pivot point be ideally? I would think it would be perfectly counterbalanced and require much less servo torque if there is exactly 50% of the area on each side of the pivot point. Is this correct?

NO!
That surface would deflect all the way one way and never come back.
A pivot at or ahead of the 25% mac works. Slightly ahead gives a bit of self-return under air loads.

I'm sure it's been covered before, but how do you determine the MAC of a swept back surface, like a jet wing?

Here's an easy way to calculate: