Frankly, stick the warning on there somewhere and move on. Californians typically don't pay attention to them anyway because they're everywhere.
Quoting myself in a previous thread:
Do you have controlled distribution channels for your products, so that they have little to no likelihood of being sold in CA? Prop 65 labeling is just concerning to me, especially if most of your customer base is outside of the state of CA. Until I started working in the food industry, I never realized how many people really do take the time to read labels (I never did - unless I was curious about the ingredients for some reason). Obviously consumers with allergy issues, diabetics, and dieters read labels, but anytime we make any small changes to our labels, we hear it from our customers. Wouldn't this cause some unexpected backlash from consumers outside of CA if the label is just placed on the product without determining whether it really and truly needs one?
As far as the question the original poster had, I thought that the recommended consumption (serving sizes) chart provided by the FDA would be the standard unit to use for determining safe harbor levels. I'm unaware of having to figure out calculations for a consumer's gross misuse of product. I could be wrong though.