As you probably know, many salts are available in forms which differ in how much water is bound up in the crystalline structure. So you can get "anyhdrous" (no water) and various levels of "hydrate", the levels depending on the salt. e.g. sodium carbonate monohydrate, sodium carbonate decahydrate.
In the case of sodium metaborate, there has been some unfortunate nomenclature which has been confusing. Lots of people don't know which version they have. Some vendors don't know what version they're selling (Vanbar in Melbourne, Australia where I got mine).
These people make it (www.borax.com) and even they have made a mistake in one of the product sheets. See if you can find it.
Some old photographic formulas don't specify which version was used.
The maximum solubility depends on which level of water of crystallization. Maybe you have the one with less water of crystallization and you already have a saturated solution. I suggest that you use that, on the assumption that it's very close to what was intended. BUT, be very careful that any particles undissolved don't get into the working solution, for obvious reasons.
Part B of PMK was always hard to dissolve completely.