First the borates:
You do not need a test kit if you are using borax to raise pH. You need a pH test! You use just like pH increaser at twice the amont by weight.
This means use about half a box for each 10k gallons, wait a day, and test pH. If it is still too low repeat. Pretty easy.
The pitting is caused by unbalance water. Low calcium is a part of that. pH is a very important part of that and low pH was probably a common problem since I am going to assume the previous owners used trichlor and only tested TC and pH (pretty common) and did not monitor TA and it dropped along with the ph many times. Combined with low calcium you have a recipe for pitting plaster. Right now your water is a bit on the aggressive side (saturation index of -.45 assuming a water temp of 76 degrees) , but otherwisey it is fine . Saturation index should be between .6 and -.6 (Some say .3 to -.3 however).
I would suggest getting the pH up to 7.6 and your water will be balanced perfectly (saturation index -.07). If you find that your pH is constantly rising too fast (because of the high TA) then you can lower the TA to 80 ppm, and raise the CH to 300 ppm after the TA has been lowered and you will still have balanced water at pH 7.6 (saturation index of -.06). However, if the pH stays stable then don't fix what ain't broke. As a general rule you want to keep the CH between 200-400 and the TA between 80-100 for a plaster pool running bleach or other unstabilized chlorine.
The two factors that have the BIGGEST influence on whether the water will be aggressive to plaster or not are not calcium and TA but rather pH and temperature in that order. Since temperature is often out of our control them the best way to prevent plaster damage is to keep the CH and TA in the recommended range (and don't lose a lot of sleep over the exact numbers as long as it's in range) and keep the pH between 7.5-7.8.
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