Borax

[chem geek]

My 21,200 gallon pool is basically 33% more water than the 16,000 gallon you based the chart on, so I need 1.33 pounds to do the same job as 1# in your chart above.

So a 4# box of borax should give me about a 1.5-2.1 bump in ph, right?

Not quite. 4/1.33 = 3 so the pH jump for 4 pounds of Borax in a 21,200 gallon pool would be 3 times that shown in the table or roughly 0.15 to 0.63 depending on TA and starting pH.