You can use The Pool Calculator to estimate the amount of Borax, but it's only an estimate.

In my spreadsheet, I put in your numbers in the Initial column (assuming a pH of 7.0 and don't forget the 22,000 volume) and then put in 7.5 in the Goal column and then clicked on the "Calculate Acid/Base/TA" button with the base set to "20 Mule Team Borax (Sodium Tetraborate Decahydrate)". I then get under Borax Inputs about 199 ounces of Borax. Since there are 76 ounces to a box, that's about 2.6 boxes.

Alternatively, if I enter in 76 ounces under Borax Inputs and then click on the "Calculate pH/TA" button, I see that one box of Borax would raise the pH from 7.0 to 7.16.

However, of more concern is why your pH is dropping so much. Usually if using Trichlor the pH will drop, but when you were using Trichlor that should have been very acidic and you should have needed to use something to make the pH rise and/or TA rise so pH Up and/or Alkalinity Up (baking soda). Now that you are using bleach or chlorinating liquid, you should find the tendency for the pH to rise, not to fall, so that is very, very strange. Something very acidic is getting added to your pool -- that is the mystery.

As Janet said, test the pH of your fill water though I would doubt that it is very acidic.

Richard