caster only has any effect on the front - it impacts turn-in (think shopping cart wheels) so playing with caster on the back doesn't do anything. note that playing with caster on the front will move the wheel in the wheel well such that it might look a bit funny and could cause clearance issues (esp when turning) if you are running max rubber.
i don't have a value for you as i kept mine stock, but check on rennlist as the 996 guys (esp gt3 folks) do play with it. here's a thread w some info ...
https://rennlist.com/forums/996-forum/1143460-suspension-adjustment-to-correct-caster-3.html