i ended up finding a reference that actually provided the half-life of rhogam, which is the product we use, and the minimum blood concentration needed to prevent immunization. it's the math behind the 28 week dose covering up to 40 weeks comes from, i dont know why neither I nor my BB manager learned this during our training!
i also pinged a senior OB I trust about this stuff, and he said she's very likely covered by what she's already got, but can redose if so desired.
So since we gave her 300 mcgs a week ago, we advised that she is very likely completely covered (she should still have almost the full dose in her system just one week out), but that OB can dose again if she feels squirrely about it.
D. Salkin, thanks for your input! sounds about like what i ended up with.