Mass Flow Rate Setting

I have a question please. In solidworks flow analysis whe you run particle tracking study what is exact unit setting of the mass flow rate. for example presume sand with 0.4mm diameter. in that case what will be best setting

0.1kg/s, 0.01kg/s 0.001kg/s or 0.0001kg/s etc. What is your suggestion please dear experts. Thank you

SolidworksFlow Simulation