I hope you have an accurate and precise thermometer. If you are off by 1 degree, you will get a 4.52% solution if you are low, and 4.94% if you are high. That's an error of 5% for just a change of 1 degree Celcius.
I have to say I think just wieghing your borax is a lot simpler and more expedient. And more accurate.
And that's assuming you have time to let your borax solution sit around so that it's truely saturated - not undersaturated and not over saturated.