Sure you are just making 100 ml of B stock solution can do it that way or like me just add the pinch to the working solution.
In any case you will be using fresh sulfite everytime and will be more are welcome.

OTOH hand it is not the working solution but the stock solution you are talking about. Please be aware of this. If you put 10.5 in the working solution you will get no stain....I promise you that.