Dave,
Having around 50 ppm Borates in your pool has two advantages:
1) It is an algaecide so helps keep algae from growing. This saves some (currently unknown) amount of chlorine loss since the chlorine doesn't get consumed preventing algae.
2) It is a pH buffer that is particularly suited at buffering a rise in pH. This will lessen the frequency of acid addition though technically does not change the total amount of acid needed to be added.
3) It makes the water have more sparkle. Not sure why this is (could be increased scattering of light).
Interestingly, the effect from benefit #1 in an SWG pool often means that the SWG output can be reduced and that lowers the amount of aeration in the pool from the hydrogen gas bubbles that the SWG makes. Less aeration means less outgassing of carbon dioxide and that means less rise in pH. The bottom line is that adding Borates in an SWG pool reduces not only the rate of rise in pH, but the absolute amount of acid needed over time as well.
There may also be some as yet unknown mechanism that causes the borates to inhibit a rise in pH by some other means, perhaps by reducing natural outgassing of carbon dioxide, but this is speculation at this point.
By the way, in the spreadsheet it seems that you forgot to set the "base" to "20 Mule Team Borax" because the amounts you quoted were for the Pentahydrate form of borate, not Decahydrate that is in "20 Mule Team Borax". For your 15,000 gallon pool, it would take 55 pounds of Borax, not 43 pounds. It would also take 53 cups of Muriatic Acid, not 14 cups. If you first enter "50" into the Goal number for Borates (with Initial at 0) and have the Base set to "20 Mule Team Borax (Sodium Tetraborate Decahydrate)", then you will see under the BORATES section that it says 883.1112 ounces so you add that in the yellow section. Then go up to the Goal and set the pH to be the same as the Target and press the "Calculate Acid/Base/TA" button. It should report 421.8479 cups of Muriatic Acid (assuming the Acid setting is set for that type of acid). I'm not sure how you got the 14 cups. Can you tell me what you did? Maybe there is a bug somewhere.
Richard