For the 1990-91 MY, NCRS uses 26 points for originality and 24 points for condition. Assuming your MY has similar points (I can check for you if needed).
If you kept the original windshield I would give you the 26 originality points and depending on the amount of area of delamination would prorate the 24 points.
As far as a service replacement I am not sure there is a standard deduction for a proper service replacement windsield for a ZR1, so I would need to check.
In my opinion unless the delamination effects the entire windshield and you can not see throught it causing a dangerous condition I would have it judged as is.
My $0.02
BTW: Tom Barr is the NCRS national team leader for our cars and would be able to provide better clarification on this issue and others.
