The one water question with an exact answer
Blending a mineral water with a mineral-free one is linear, so the split is arithmetic. What the target should be is a preference, and this page says so.
Exact split
Exact for the mineral you measured. It says nothing about taste, safety or any other property of either water.
Brew the same coffee with your tap water and with a low-mineral bottled water and compare them. That single experiment tells you more than a table of ions, and it costs one bottle. The water guide explains what hardness and alkalinity actually change.
No target is recommended here, no drinking-water safety assessment is made, and no equipment scaling advice is given. The SCA standards programme publishes water standards for coffee; your machine manufacturer's water requirement overrides anything on this page.
Blending is linear, so the share of your own water equals the target concentration divided by its measured concentration. The calculator gives the exact split for your volume.
Not on its own — water with essentially no minerals tends to brew flat and hollow. Distilled water is the base you blend down to a target with.
CupTune does not recommend one. Published standards exist behind the SCA standards programme, and the practical first step is comparing your tap water against a low-mineral bottled water by taste.
No. It performs a mass balance on the one measurement you entered and makes no claim about drinking-water safety, equipment scaling or any other ion.