This site says that the Formulary's divided D-76 contains 60 g/L of borax, which agrees with your numbers.
But my notes say that the solubility-limit of borax is 47 g/L (but I didn't record at what temperature).
This site gives an even lower limit of 25.2 g/L.
ChemicalBook here gives a limit of 26 g/L at 20 degrees C.
Even adding in the weight of water of hydration (decahydrate), the solubility limit goes up to only around 47-50 (agreeing with my notes).
Your results are consistent with exceeding the solubility-limit.
I don't see how that formula can work. And the Formulary is selling it? I'm mystified.

Mark Overton