Kirk, if the saturated solution has been decanted from the undissolved sediment at 20 C, it will contain 47.1 grams of borax per kilogram of solution at any higher temperature. For practical purposes, even if you measure by volume at a higher temperature, the change due to expansion of the liquid will not throw you off as much as the possible variation in weight per mol of Na2B4O7 due uncertainty of the average amount of water of crystallization. How do you ascertain that number? Every time you open the container, the contents are subjected to possible changes in humidity. The 5 pounds I bought from the Formulary has huge lumps, small lumps and powder, which indicates changes in water of crystallization have occurred, but from what to what?
If you are going to be a stickler for precise measurement of borax, your best bet is a saturated solution. I think the best way to make it is to start from saturation at a higher temperature and to let it cool to the desired temperature. If it takes a while, so be it. as the saying goes, "Think ahead."
I did not continue my formal study of chemical engineering, but changed to aeronautical. I am an engineer by nature, as anyone who knew me during my years at NACA-NASA would testify. As one of our instructors in engineering was fond of saying, "Engineers can do anything."