How do you really convert serum HBsAg measured in ng/ml to iu/ml?

In your paper, it was stated : "For convenience, HBsAg concentrations measured in ng/mL
were converted to IU/mL, considering that 0.15 ng/mL of HBsAg is equivalent to 350 IU/mL."
This seems to vary significantly from the usual conversion rules of:
1. 1 ng/ml = 0.2 iu/ml or
2. 1 ng/ml = 1 iu/ml