purity of drug standard on anhydrous basis.

Dear Dr,
If i have one drug standard with having purity of 100.00 % by HPLC. and water content 0.4% by karl fisher.
if i want to convert the above purity on as is basis than shall i use below mentioned formula??

100-water content x given purity / 100= as is basis purity.

example:

100-0.4 x 100.00 / 100=99.60% on as is basis.

Kindly reply me ASAP.

Thnaking you...

Gaurav