MSU Rain mix with tap water - Calsium issue

[Jago]

Hey,

I ordered the Akerne Rain mix.
Upon receiving it, I realized the CA content is quite high. Reading more into it, I found out this is actually for use with rain water (Name should have given me some indication, perhaps)

I live in the Netherlands and the water here is very rich in Calcium.
I might be able to start collecting rain water too, but ATM I only use tap water filtered through a Brita can.

Do you think the Brita can remove enough calcium?
Would the excess calcium be a problem?
Could the precipitation effect remove enough CA from the mix?

My medium is just bark and coal, with leka drainage. The bark is starting to break down, so maybe that would lead to more acidity to compensate for the CA(?).
(My orchids are mostly Oncidiums, with some Paphs and Phales)