Is there a standard or guideline to this? For instance, is there a prototypical person with an average weight they design dosage parameters for?
It varies; however in general it is based on a healthy individual ages 4 - 50 consuming 2,000 calories a day.
The LiveStrong web site states:
There reference to the USDA allows us to get a much more granular view and covers the varying life stage groups.
If you take a life stage group and what the average weight is for that given group, that would define your prototypical person within that category that they base their figures on.