OK, so before the water change yesterday:
Ca: 432
KH: 5.94
Did an approx 30% change yesterday. Exactly 24hrs later (yep):
Ca: 484
KH: 6.72
Mg: 1440
The baseline KH of the mixed water is 8.9.
I'm going to test Ca and KH again each night for a week, and see how they trend.