If you had prepared a solution of a standardized reagent, what would be the best to prepare a receiving flask?
The best measure would be to flush the receiving vessel with a small volume of the reagent.
You have a reagent of known concentration, and you want to put it into another vessel. Even if the other vessel is completely dry there may be a residue that would react with the transferred reagent. So sacrifice a small volume of your reagent. Pour in a few mLs, rinse and shake the container, and then repeat the process. Then refill the container with your standardized reagent.