Dear Justin,
I looked into the BBP rule and did not see the 75% rule mentioned. It may be a best practice or a manufacturer's use rule. Assuming the 75% rule is mandatory (regulatory or manufacturers instruction), I would follow it and assume the 40 gallon bag is an additional layer of protection.
If it is a best practice, then can the 40 gallon bag serve the same purpose? If so, why have the sharps container at all? If not, then I think you're back to 75% rule.
MikeCJ