You can use The Pool Calculator to calculate dosing where 50 ppm Borates in 13,000 gallons would be 496 ounces by weight or 534 ounces by volume of boric acid OR is 765 ounces weight (about 10 76 ounce boxes) or 734 ounces volume of 20 Mule Team Borax with 365 ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid).

Borax plus acid is less expensive, but boric acid is more convenient since it only lowers the pH a small amount (roughly from 7.5 to 7.2). If you use Borax plus acid, add one-quarter of the total amount at a time, starting with the acid first adding slowly over a return flow, then add the Borax, wait 10 minutes, add acid, then Borax, etc.