Carbonate will absorb water in storage, but usually the anhydrous form is specified. Since it is possible for some or all of the molecules to bind with water it makes no sense to specify the hydrate. That said, the fact that there is water binding to the molecular forms dissapears in solution. The only reliable way to determine the amount of carbonate needed is through measurement of pH in the solution. Once you determine the amount needed of your stock, then you can reliably mix by weight, but for your stock only. Once mixed you should properly adjust pH as needed using either base or acid to make pH corrections.
If you are scratch mixing solutions then you should be measuring pH.