Rudeofus, thanks for the link to that older thread on this topic. That thread implies that the Formulary started getting another kind of Borax which caused problems. I just did some searching, and the only kind of Borax I see on the market is the decahydrate. Both borax.com and Spectrum Chemical (the Formulary's supplier) sells the decahydrate. I wondered if the formula wanted something with higher hydration, but I can find no such thing. Well, there's a pentahydrate, but that should have even lower solubility. And yet formulas call for over 50 g/L? I'm still mystified.
As a prior poster has said, your Bath B should work fine. My concern is that tiny particles of undissolved Borax might get embedded in the gelatin of the emulsion, creating tiny spots. I suggest cooling it to 12-15C, filtering well, and warming to 20C (the normal temp) which hopefully will cause any residual particles to go into solution.