The "flight visibility" referred to in 91.175 is the reported visibility from ATIS, AWOS, Tower, or other ATC/FSS facility, and must be met to begin the approach. Once you get to the DA/MDA, the "visual reference" criteria of 91.175.c.3 apply, per your SOP.
Assume the reported visibility was sufficient when you commenced the approach at the IAF or took your first vector to intercept the approach. You can assume that visibility is still valid unless ATC specifically tells you (or broadcasts on your assigned frequency) the visibility has been reduced. Once you pass the FAF, you can continue to the DA/MDA regardless of any reduction. You can then assume the actual visibility is sufficient if you meet the "visual references" requirements -- THAT is your assessment.