More is better for return steering
The correct answer is as much positive caster as the car will give you, with ZERO camber. I have been able to get 5 deg on some and a little less on others. If you could put it at 8 deg that would be heading in the correct direction but you likely will not be able to get that much. Just remember that you have to have the same degrees of caster on both sides and have zero camber on both to go strait.
Bruce