If it is washing soda it is not monohydrate nor is it anhydrous. Washing soda is the official common name for the decahytrate. That is why you must use 2.3 times as more by weight than the monohydrate, or 2.7 times more than the anhydrous.
If you are not sure what you have, you can heat the heck out of a weighrd sample of it. If it loses weight it was not anhydrous. If 270 grams shrinks to 100, it was washing soda. If 124 grams shrinks to 100, you have monohydrate.
I'm sure you can do the arithmetic for other amounts.