Strictly speaking my math was correct. The conversion from X mg/kg to Z Mg/sq m was correct. 200 mg/kg times 37 kg/sq m = 7400 mg/sq m.
However, I did not divide 7400 mg/sq m by the dimensionless number 12 to get to 617 mg/sq m. The dimensionless number is used to approximate the conversion from mice to humans.