I don't think we need to use the molecular weight for anything, unless we need to determine molarity, as an example.
Moisture is a dominant impurity, the assay tells you how much added weight the sample has, so you should use proportionally more. Warning: on this planet, moisture is everywhere. The value may change. You may be called to determine moisture content, by Karl-Fisher titration, just before using this reagent. You will find such regulations in The Merck Index, or whatever pharmacopeia you're using.
You might also want to try to figure out what the assay results mean, and if they're significant for your application.
You can also just prepare a 1 mg/Ml solution, then, when you have results, scale them according to your purity. You will probably have to do this anyway. Consider: How would you prepare 1 mg/mL of a substance you knew was 100.0000% pure? Would you weigh out 1.00000 mg? What if you try, and get 1.00010 mg? Are you going to remove and discard 0.0001 mg?