Nano-doses are lower because the particles in nano-form has large surface area to react than normal . To calculate the dose accurately you have to test it under different concentration, from that you may achieve the exact doses for maximum benefit.

I agree with the answer given by Tarafdar. The lower the particle size, the higher the surface area, and the higher the rate of dissolution and rate of absorption that often results in better bioavailability. Generally, nano doses are very potent compared with others.

Site delivery options which also decides the nano-materials reach with varying factors in transport (stability, degradation etc.) and the carrier payload deliveries accumulating (actual on site bioavailability) the concentration at the receptor site (usually obtained through plasma concentrations) are other factors to get in calculating the required dose.