How to calculate the quantity of required quantity depending on potency and water content.

Are you asking how to calculate the clinincal dose from known IC50, EC50 and LD50 values?

i mean in pharmaceutical formulation, i need to make the compostion of each ingredient to build the formula, there are some active ingredients need to be adjusted each time depending on the purity of supplied raw material which is affected by impurites and water content.

You mean allocation based on potency. There are 2 ways of doing this. One involves figuring out how much of the active you will need per lot based on the potency and adjusting the dosage weight by adding or subtracting an amount of an inert excipient so as to maintain the original dosage weight. the other way is to figure out the amount needed and adjust the dosage weight accordingly.

Are you making a tablet?

yes I make a tablet, dry powder for reconstitution.
but the problem what equation should I use to calculate the required amount of API to put in BOM(bill of material) based on water content, the assay of API (either as is or on dry basis).

Just divide the theoretical amount required by the potency on an as is basis and you will get the required amount.