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I can help you here. You are very close (in your understanding) ... If you are a spouse or survivor, then the GPO (Government Pensioin Offset) reduces your benefit by 2/3 of the amount of your CSRS pension. (Which, if large enough could eliminate the Social Security benefit.
For the worker's benefit itself, there's something called WEP (Windfall Elimination Provision) which is a more complicaed formula - Essentially this reduced the 90% factor (there are three of them) of your AIME (Average Indexed Monthly Earnings) to 40% at 20 years of social security.
(Once you have 30 years, there is no reduction at all).
So with WEP, you'll never have every penny wiped out. With GPO (for survovirs and spouses) if the social securit is small enough ans the non-social secuity benefit large enough, it can wipe out the social security benefit completely