Magnesium and ratios ... 5:3 vs 8.5:1

[Delphinus]

Ok, so I understand that you normally want to dose Magnesium at 5:3 for MgCl / Mag sulfate, or 8.5:1 if you run a calcium reactor.

I recently started using a reactor again so it makes sense to me that to dose Mg into my tank that I want to use the 8.5:1 ratio.

However what about the make up water for water changes? I usually like to top up this water for Mg (I use IO and Mg is a bit low for my liking). Should this not stay at 5:3?

Thoughts?