The PoolMath calculator is handy and validates my point. For a target dose of 50 ppm "borate", PoolMath suggests adding 4582 oz of boric acid to one million lbs of water (120k gal), which comes out to 286 ppm boric acid. Boric acid (H3BO3) is 17.5% boron by weight, and 50 / 0.175 = 286.
What is confusing is that the PoolMath page calls it "borate", but based on my test here, is dosing as boron. (Borate actually means the BO3 ion.)
I think the current treatment rate of 50 ppm is not really a magic number for pool chemistry, but is based on EPA and NSF assessments that determined this level doesn't pose a risk to human health. Their analyses were based on concentration as boron, not borate. For example, see this NSF document.