When you weigh your reference standard and use the mass in the assay calculations of your formulation, you must correct for the water since the reference standard is the di-hydrate form. You know the molecular weight of Lisinopril and of the two H2O molecules so you would apply a mass correction factor of M.W. anhydrous form/ M.W. di-hydrate form.
Using this gives the Mass of anhydrous Lisinopril in the Reference Standard Portion that was weighed.