Yes,keep adding it--it's too much when your pH goes over 8.0--seriously!!
Having a pH below 7.0 can do some significant damage to your vinyl, so it's critical that you get it above 7.0 ASAP--so whatever amount of Borax it takes to do so, that's the correct amount.
You have already removed the remaining trichlor pucks, right? If not, then you need to do so--that's your major culprit for the low pH.
Janet
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