Working standard qualified against primary reference standard.
Reference standard are available from US pharmacopoeia or European pharmacopoeia as per your market requirement.
Simply performed assay against reference standard and calculate Potency .
For Example
Assay is 99.5% on dried or anhydrous basis
water content or Loss on Drying is 0.5 % of API
calculate Potency (100-0.5)*99.5/100
Potency as such = 99.0%