Don't add all that borax at once, you don't want to overshoot. pH change is very difficult to calculate and non-linear. Add a couple boxes, let it mix for a couple hours, measure pH, dose half as much, ... repeat as necessary. It's fine if you want to use the calc as a range for purchasing the borax. You can add borax s-l-o-w-l-y through the skimmer, it will form a hard plug if you add it too fast. Break up any chunks as you go.

The water will leech calcium from your new plaster, I'd get CH up to at least 200.