Drilling time: t=L.N/n.f

L is hole depth (mm), N is the number of drilled holes, n is rotary speed (rps), and f is feed rate (mm/rev). Accordingly, drilling time is obtained.

After that, measure flank wear of drill bit.

Finally, wear rate:

Wear rate= Flank wear/Drilling time

This method is more common in calculation of drill wear rate compared to weight loss measurement. But in your method, do as follows:

Wear rate= Weight loss/Drilling time