Borates don't reduce the amount of acid you need to use (assuming TA and aeration levels remain the same). Total acid usage will remain the same. They reduce the rate at which the PH changes so you can go longer between acid additions
However, Borates allow you to aim for a lower TA level than you can have without borates. Since the borates act as a PH buffer you don't need as much PH buffering from TA. If you do lower the TA then the total amount of acid needed can be lower than without borates.
You can use my Pool Calculator, see the link in my signature, to calculate the amounts of borax and muriatic acid to use.
I suggest adding one gallon of acid, then 3.5 boxes of borax, and repeat as many times as needed. At the end you need to make sure the total amounts of acid and borax added come out to the amounts calculated, so the final round is fractional and requires more careful measuring. Then circulate for a couple of hours and then check the PH and balance as needed.
You don't want the PH to swing around any more than necessary, so I like breaking it up into smaller units than simply half and half. One gallon to 3.5 boxes isn't the exact right ratio, but it is fairly close and reasonably easy to do. It is also important to remember the final PH check. Any mistakes can leave the PH way off, so it is important to double check that the PH came out right and to fix it fairly quickly if it is off.