A 20% water change of water that has a Alk of 9 would take a tank with an alk of 8 bringing it up to 8.2.
A 20% water change of water that has a Alk of 13 would take a tank with an alk of 8 bringing it up to 9.
This is part of the reason that I say any salt will work fine since a water change is not highly impactful to overall alk increase.