How much of my retirement is my ex entitled to?

[Concerned in Chino]

We were only together for 3 1/2 years. How much of my retirement is she entitled to and how much of hers (which she has already cashed out) am I entitled to? If I have overpaid her for the difference in value of our accounts, can I get that money back or at least deduct it from my child support payments?

Thanks from Calif.