I got the ANSI specs for Photography chemicals sodium tetraborate, pentahydrate and decahydrate, 3 pages ata cost of about $21. In a nutshell, the summary of requirements is:

Assay, Pentahydrate or Decahydrate......101% min, 105% max.
Heavy metals (as Pb)......0.002% or 20 ppm.
Iron (Fe)......0.003% or 30 ppm.
Carbonate......To pass test.
Reaction to Ammoniacal Silver Nitrate.......to pass test.
Appearance of solution........to pass test.

The tests are described in great detail. Kirk or PE could perform them. Most of us have not the equipment nor the skills to perform them.

It appears that the specs would be met or exceeded by the NF grade described at www.borax.com, and the SQ grade would come very close.

You will notice that the minimum assay is greater than 100% and less than 105%.