In low huidity it shouldn't take much heat, but a lot of tumbling, to change the deca to penta. Whatever you start with, it's just as easy for me to put enough in a pot of water so that not all dissolves, decant the clear liquid, and note the temperature. As long as you keep it at that or higher temperature, your solution has a fixed % of the equivalent of sodium borate decahydrate. If you can't find the chart at I can send you a copy.

Metaborate is another conundrum. It has two molecular weights which are identified as 4-mole and 8-mole, leading one to believe that these are states of hydration. However, one ghanges to the other in a closed container above a particular temperature and back again as the temperature reduces, obviously with no change of water content, so it is only of academic interest.