Power steering caster can, and should be much more positive than manual steering. If the manual spec is 3, for instance, start by doubling it, and add more if necessary (top of the king pin pointed more toward the rear). Trying to make a power steering gear box return to center requires much more effort than the same steering system with a manual box. Turning away from center lifts the front end of the vehicle, and the weight pushing down on the spindle is what causes the return to center. Power assist makes the "lift" easier, so much more return force is available with increased caster.
There's a problem called "caster scrub" that happens when excessive caster angles are reached. Its symptoms are increased tire wear with a feathery appearance of the tread. It takes LOTS of caster to do that, however. I've seen that problem on European cars such as some big Mercedes models where the specification is +9 degrees or so.
Jerry
