I am currently working with a drug molecule known as Amiloride $\ce{HCl}$ and it seems to be a non-stoichiometric hydrate.

How would I go about making a solution with a specific concentration of Amiloride $\ce{HCl}$? I know its anhydrous molecular weight but if I don't know how much water is in the solid crystals how can I get a specific concentration?