Patrick, the 5% figure came from Kirk's post and suggested the limits of a 1% error in temperature either high or low. That is where it came from, nowhere else. In my experience the Borax from the Formulary is just fine.
As for all of this work, it seems to me that there are two considerations...
1. How much does this purification cost in terms of time and energy? Is it cost effective? It actually is not apparently cost effective as was described in an earlier thread I believe. I did not make the post nor have I tried to locate it or do the estimation myself. The very proposition of purifying my chemistry before I use it is alien to my thinking and work flow.
2. Should I even worry about borax? It is one of the chemicals that Kodak worked very hard to eliminate in the 60s and 70s and resulted in a whole new family of developers which used carbonate, TEA, or very low levels of borax to achieve superior results. The reason? Borax is toxic to some plants, some bacteria in sewage systems, some insects, and small children. Kodak wanted to become greener. So, I seldom use Borax.